She's a young lawyer on the Hill, and when she fell in love with a man with three children, she didn't really think about them as being part of her life. He spent at least part of every weekend with them, and sometimes they spent the night in his apartment while she went back to hers. They were always polite to her. But the first weekend after she moved in, they got as far from her room as they could get. They were crying. Still clutching their overnight bags, they told their father, "We want to go home."

That was six years ago. They haven't spent the night since.

This young woman -- independent, bright, attractive, and used to getting what she wanted -- had become a Stepmother.

"I wasn't prepared for how much I would feel like an outsider," she says. "When they weren't there, I was everything to my husband. When they were there, I had to share him. I felt threatened because I knew the children came first."

Stepmother. The word itself resonates with negative connotations, summoning up images of the wicked stepmother from Greek tragedy to Cinderella. The modern stepmother feels alone, though her ranks are growing, especially among women who postponed marriage. Some have entered a second marriage with their own children -- and his; some have become full-time stepmothers, others are weekend parents. According to the Step-Family Foundation in New York, by 1990 more Americans will be part of a second marriage than a first, in large part because of the high divorce rate. It also says that one out of every six children lives in a step-family.

Abraham Lincoln had a stepmother. And now, there's a stepmother in the White House, who seems to be having problems like everyone else.

It should be no surprise that becoming a stepmother is so difficult, since the relationship concerns the Big Four issues: love, money, power and sex. Time and again women interviewed said that becoming a stepmother was the most challenging experience of their lives. Their expectations, shrouded in innocence or ignorance, were unrealistic, and their frustrations were soothed, in most cases, only by the passage of time.

"It was the worst year of my life," said Fran Hattin of the first year of her marriage, when her 14-year-old stepson came to live with them.

"All I can remember is a total feeling of exhaustion and disbelief," said Susan Kulp, who combined her three children with her second husband's two in what is called a "blended family."

Another woman, who asked that her name not be used, said, "When I first got into this, I was so concerned about the children -- that they would be going through some trauma, that they would feel uncomfortable, or resentful or whatever -- it never occurred to me that I might be traumatized."

The statistics: Fifty million Americans are currently involved in step-relationships, and roughly 750,000 women a year marry men who have been married before. Journalist Glynnis Walker's survey of 200 second wives found that 84 percent of the husbands had children from their first marriage. In her survey, 60 percent of the first wives did not work, while 74 percent of the second wives did.

Marriages, experts say, dissolve for a complex set of reasons, but Ann Landers believes that children are the biggest single cause in the breakup of second marriages. In Walker's survey, 35 percent of the women said that the children of the first marriage are the biggest problem a second wife should be prepared to face (26 percent said it was the ex-wife).

"I wouldn't say that breakups are solely the child's fault, but often the relationship is not strong enough to survive the onslaught of the child's anger and manipulation," said Hattin, a social worker with the child guidance department of the Jewish Social Services Agency. "A child can cause such havoc . . ."

(P.S.: Her marriage survived.)

Psychologist Nancy Hafkin, a cofounder of the Stepfamily Association of America and the wife of a stepfather, added, "In first marriages the issues tend to be in-laws, religion and sex. By the time of the second marriage it tends to be finances and children. Children don't break up a marriage, but if the problems they create aren't dealt with, people can get so unhappy they can't stay in the second marriage."

Sandy Nichols, an independent filmmaker, was 31 when she met Mike Edwards, a widower with two daughters, ages 7 and 8. They married two years later, in 1978. "Our first date was a blind date with each other," she said. "Our second date involved the kids. They came and picked me up and we went to see 'King Kong,' and the two kids made sure that they sat between us."

Julie Hubbard was Henry Kissinger's private secretary when she met Henry Hubbard, then covering the Nixon White House for Newsweek. She met his three children, then 5, 8 and 11, on their second date. "I think we went to the zoo," she recalled. "His daughter called me by the previous girlfriend's name the entire day. Now you can say, oh well, she was only 5, but I don't buy that for a minute!"

In Nichols' case, the girls were friendly and encouraged the romance. Hubbard took five years to get married, largely because of concerns about the children.

"When Mike would pick me up for dates, sometimes he would bring me little notes from the kids, asking how I was or just sending love," Nichols recalled. "There was a real friendliness and outgoingness at first . . . Once we all went off to the Grand Canyon together. The kids were in the back seat, and at one point one of them pipes up, 'Sandy, if you and Daddy get married, can I be a flower girl?' We hadn't even discussed marriage yet!"

"I was pretty young 24 for taking on those kids, and they were old enough to resent me," said Hubbard. "The boys' resentment was not directed so much at me; I think they were angry about the situation their parents' divorce . They were old enough to know what they'd lost . . . I always had sort of rosy expectations, and I would attempt to create things right out of storybooks about it being so nice for all of us. But the truth is that it's not nice for them a lot of the time. They missed their father dreadfully. And it didn't ever make it better that I was there. Probably not ever."

While a stepmother may see herself as a phoenix, rising from the ashes of a dead marriage to create a new life, the children see her as symbol of their destroyed home. She represents their hurt, the fact that their father isn't living with them anymore, or the death of their mother. No matter what the child's age is, whether 3 or 30, resistance is almost inevitable. This resistance ("acting out" in psychiatric parlance) can range from reserve to rudeness to more brutal forms like getting involved with drugs, running away or wrecking a car. Whatever it is, the strains on the new marriage are such that only the strong survive.

"The step-family initially might be considered analogous, psycholgically, to what physically occurs in organ transplants," says a brochure published by the Stepfamily Foundation. "The body responds protectively and rejects foreign tissue."

To be sure, there are deliberately wicked stepmothers in real life, misbegotten souls who reject or disinherit the children, or force them to wear hand-me-downs while their own children wear Izod T-shirts, or impose household rules that would make Capt. Bligh look like a pussycat. Or, worst of all, force their husbands to choose them over the children.

But by and large the modern stepmother is the beneficiary of a generation of psychological awareness. She is thrown by coincidence into a role that is neither friend, nor mother, nor aunt nor godmother. If her husband is divorced, the world suspects her of being a home wrecker; if he is a widower, the second wife is invariably a pale successor to a saint. Often there is a bitter or vindictive ex-wife in the picture.

Suddenly she may find that her weekends and vacations are invaded by someone else's children. Her salary is being spent on milk and pizza instead of silk blouses, and the house or apartment that was fine for two is littered with toys and dirty socks. Her Prince Charming may turn into a frog, occupied with their demands; and their sex life may disappear into a vortex of tension and lack of privacy as soon as the children cross the doorstep. She may find herself turning into a drudge, cooking and cleaning for little reward.

Sometimes she must postpone or defer having her own children to help support children whose natural mother refuses to shoulder financial responsibility. If she has a child, financial pressures may force her to return to work sooner than she might like. Yet not even greeting card companies acknowledge her existence, and her public image is universally dismal.

Humorist Garrison Keillor, writing in The Atlantic of a mythical "Weeseville, Pennsylvania," tackled the subject this way:

". . . a woman was dismissed from her job as a human resources coordinator and driven over a cliff by an angry mob of villagers carrying flaming torches and hurling sharp rocks after they learned she was married to a man who had custody of his three children by a previous marriage," he began a piece in which Cinderella, Snow White and Gretel reveal that their stepmothers got a bum rap.

"My God," said one stepmother of a 12-year-old boy, after laughing at the story. "That's exactly what we're all really afraid of."

Life with children is often made up of subtle moments. The way his daughter always manages to prevent you from sitting next to him. The way his son doesn't like anything you cook. The little comments: "You should see how fast my mother can peel carrots." "My mother always leaves the butter out, so it's soft."

"I would find myself literally counting the hours until they left," said Hubbard, who spent many weekends with the three children in a cabin in the Blue Ridge Mountains. "They still talk about how astounded they were that I was so strict about table manners. I thought they were really out of control. I had a very Victorian kind of upbringing, and having to deal on a regular basis for a full day with kids who were really pigs was distasteful to me . . . I'm sure they thought I was a beast."

Jeannette Lofas, in her book "Living in Step," coauthored with Ruth Rosenthal, said she would actually start feeling nauseated the morning her four stepdaughters were due to visit. "They would come in and hardly say hello. I would reach out to them; they would walk away. I would hug them; they would stiffen. I would speak; they would say little or nothing. The got me where it hurt the worst. The more I talked, the less they responded."

These woman shared a problem that experts say is all too common: unrealistic expectations. "Somebody who marries a man with children tends to be a nurturing, caring kind of person who wants to make a difference in someone's life," said Nancy Hafkin. "They enter step-parenthood thinking all they have to do is love everyone and everything will turn out fine."

Not only should a stepmother not expect to love or be loved, experts caution, she should not even expect friendship -- at first. She should expect nothing.

"You have all kinds of fantasies that aren't right at all, of course," said Hubbard. "That they'll sort of look up to you. That you'll be a sort of nice friend. It ain't so!"

It was her first Christmas with the three stepchildren, and she'd taken a great deal of care on their presents, choosing gifts that were not embarrassingly expensive but showed thought about each child. As they sat before the Christmas tree, decorated with bulbs and ornaments she had purchased, she had her first inkling of what was about to happen. They began distributing the presents, and after about half an hour it became clear. She wasn't going to get any.

As the wrapping paper piled up around everyone else's chair and she sat there surrounded by yawning, clear space, she felt as though a neon arrow was pointing at her from the heavens, with a big sign that said, "This person is being rejected."

Finally a book from her husband was unearthed, and a box of ugly notecards from a neighbor. That was all. She felt like Cinderella instead of the stepmother.

She spent the afternoon cooking a wonderful Christmas dinner -- her first -- and each dish was greeted with pursed lips and suspicion. "What's that?" asked one of the boys, peering at the Yorkshire pudding as though it was formaldehyde. "Where's the candy?" demanded her stepdaughter. "We always have candy in little dishes by each plate."

After dinner, and as many glasses of wine as she could grab, she went up to her bedroom and closed the door. She was too exhausted to cry. Is this what it was going to be like for the rest of her life?

The idea of the cruel stepmother has been honed over centuries, as Brenda Maddox discusses in "The Half-Parent." Greek mythology contributes several horrors, and Shakespeare's Imogen refers to her stepmother as "Queen Poisoner." The stepmother in "Snow White" tried to poison her four times; Hansel and Gretel's stepmother forced their father to abandon them in the woods. The Cinderella legend has been traced to 9th-century China and one scholar has found 345 versions of the story. (The word "step," Maddox wrote, comes from the Old English "sto-ep," which is connected to bereavement.)

Horrible stepfathers are amply represented in literature (in "David Copperfield," for example), but only stepmothers get the bad rap in children's stories, Maddox notes. She speculates that this may be because mothers spend more time with children. Since hating your mother is blasphemous, "bad" mothers -- stepmothers -- are invented in order to vent this anger, she says. Furthermore, until inheritance laws were changed, there was a legitimate fear of a stepmother preventing a child from inheriting his father's estate -- especially if she had a son of her own.

In Latin, stepmother is "noverca," which was also used as an agricultural term meaning ditches that drain off water slowly; or rough terrain.

Rough terrain: One man recalled the stages of his relationship to his stepmother, who married his father a year after his mother died: "Suspicion. Fear. Then, when it became clear that she was here to stay: horror. Then resentment, guerrilla warfare, followed by sullen acceptance. Grudging respect. Admiration, and finally, genuine affection." And it only took 15 years.

Tomorrow: Stepmothers discuss their failures and successes CAPTION: Illustrations 1 and 2, Two of the most wicked stepmothers appear in Walt Disney's features "Snow White" and "Cinderella;" Illustration 3, The Wicked Stepmother and her spoiled daughters in Disney's "Cinderella." Illustrations from Movies Stills Copyright (c) Walt Disney Productions