Joan Yates putters around her aging Accokeek, Md., rambler, wearing second-hand dresses she bought more than a year ago. Sometimes she, her 8-year-old son and new husband can't afford enough food to eat, so her parents have to help them out, she said.

Her husband, Allen, wants to adopt his stepson, but he says he can't afford the $250 in legal fees to do so. Yet the family's $10,000 yearly income is too high for food stamps or free legal aid.

The problem is that Yates must support two families: his present one and a daughter from his first marriage, which ended in divorce last year.

With one in three marriages ending in divorce, a growing number of "second wives" and "second families" are becoming bitter about divorce laws that place the needs of a divorced man's first family ahead of the needs of the second family.

But legal experts say the first family is entitled to support from the father and that the American Dream does no necessarily include the luxury of supporting two or more families.

"We've gotten to the point where we just can't make it," Mrs. Yates said. She talked at length about how hard her husband works and said she is not complaining to get revenge on his first wife. But nobody can help her, she said. "This whole set-up is so unfair to the second family."

To console families and lobby for changes in divorce laws and the way judges interpret them, organizations such as Step-Parents Forum in Canada and Remarrieds, Inc., in California are being formed.

Some of the groups, composed mainly of women, have stormed the chambers of startled judges demanding they require smaller child support or alimony payments from husbands, neighbors or friends.

Elliott H. Diamond, who coordinates a fathers' rights group in Virginia and the District, is helping a dozen Virginia women start Second Wives, Inc. A chapter already exists in Baltimore. A national group, National Committee of Second Wives, is based in New York.

But even Diamond concedes that there is little these groups can do.

Margaret Brooks, a Falls Church woman, said she began working as a secretary after being a housewife for 15 years in order to help her husband meet his $250-a-month child support payments to his first wife.

"I talked to some women with the Second Wives in Maryland," Mrs. Brooks said. "But even some of them said there really wasn't much they could do when I asked them how they could help me. They say they're trying to change the laws. I guess it's just nice to have someone to talk to when you start feeling bad."

Doris Freed, chairman of the American Bar Associations family law section, said men like Yates knew what they were getting into when they assumed responsibility for a second family.

"There is no way that he can or should escape the obligation of his first batch of children," Freed said. "To me this does not seem unfair. To me parents are forever.

"A man has a responsibility to his first wife and his first family," Freed said. "Just because he's divorced his wife doesn't mean he's divorced his children. When he gets married (again) he knows he has this burden."

Yates said he does not dispute that he has an obligation to his daughter. But he says his child support payments of $180 a month should be shared by her mother, who lives in Virginia. Under Virginia law the mother is not obligated to work to receive child support.

"It's his child as well as mine," said Betty Yates, Yate's first wife. "I've put out more than he ever has.

"I've cared for her through sickness and everything else," Betty Yates said in a trembling voice. "If he thinks that takes money he's wrong. It takes a lot of love and a lot of worry. The judge has already told him what to do," pay $180 a month in child support.

After taxes, Yates, a manager for Chantilly Freight at Dulles International Airport, brings home about $7,000 a year. A Fairfax County judge last September ordered Yates to pay the $180 a month to support the 15-year-old daughter of his first marriage. Yates then is left with about $400 a month to support his second family.

The judge told Yates last June, when he requested that his payments be reduced, that if he couldn't make ends meet with his second family, he should get a second job, Mrs. Yates said.

Mrs. Yates said she and her husband were shocked at the suggestion that he get another job. He has not. She said she wants to help her husband, but he does not want her to work. Mrs. Yates said, "I don't owe her (the first wife) any money. What I would make I don't feel" should be used to support Yates' first family.

To get child support or alimony in Virginia a woman must file a petition in Juvenile and Domestic Relations Court.

When judges decide on amounts of child support and alimony they are following guidelines set forth in the Virginia code, according to Alexandria Circuit Court Judge Donald H. Kent.

A recent revision in Virginia's divorce law requires judges determining support settlements to consider the earning capacity, obligations, needs, financial resources, education and ability to secure education or training of both spouses. Also considered is the duration of the marriage, standard of living established during the marriage, the spouses' ages, physical and mental conditions and monetary and nonmonetary contributions.

In alimony decisions, "the court will not seek to find how light the burden may possibly be made, but what, under all the circumstances, will be fair and just allotment," the law states.

The husband's capacity to earn is especially emphasized, not his current income.

Kent, who last May ordered a mother to make child support payments to a father who had custody of their three children, said there is no set rule to determining support payments.

"It depends on the circumstances of the case" Kent said. "Some people can afford it, others cannot. Each case must stand on its own merits.Each case is different."

But what irks second wives like Mrs. Yates is that divorce laws ignore the expenses of the second family, although she and her son are enlisted as dependents on federal income tax forms. The courts consider only the husband's expenses in computing support and alimony payments regardless of the needs of his wife or how many children he has.

"People come in all the time. They say I have two kids. They're going to starve if they have to pay $200 a month," said a Fairfax attorney who frequently handles divorce cases.

"They try to raise two families out of one income," the lawyer said. There's no way to do it, even in Fairfax County," which is one of the most affluent counties in the nation.

"If the wife says she wants $1,000, the father says he can afford $500, the judge says $700," the lawyer said. "Both sides leave unhappy."

The attorney said women's liberation, with its theories of equality for men and women, have fueled men's minds with thoughts that their first wives will share in the support of their children.

But it usually doesn't work that way.

"It doesn't make sense. We feel (the first wife) should be equally responsible," Mrs. Yates said. "We're just interested in seeing equal justice."