Q: I've been married just over two years. We have no children of our own, but my husband has a 16-year-old son and a 10-year-old daughter from his first marriage. Since they live about 1,000 miles away, we see them infrequently but my husband keeps in close contact by telephone. My relationship with the kids has been good.

My concern is a growing problem with his son. He is a classic underachiever. For the past 18 months, he has been cutting class, on and off, and may not graduate with his class. He has switched high schools at least four times. His mother has tried family counseling, but he refused to attend. Despite all this, he is basically a good boy and there is no sign of drug or alcohol problems.

He seems to be working toward an all-out confrontation with his mother so that she will get fed up and send him to live with us. He seems to have an idealized picture of what life would be like with us.

My husband asks me with increasing frequency what I would do if his son just turns up on our doorstep one day. I like my stepson but doubt I am ready for instant motherhood. We are close to starting our own family and I worry about what would happen with him living here.

Another concern is my stepdaughter. She is a nice girl and a good student, but I'm afraid she's learning that the best way to get both parents' attention is to misbehave.

I have discussed my concerns with my husband. I'm worried we'll soon be facing a major decision.

A: Don't count your calamities before they're hatched. Teen-agers have always threatened to leave home and mothers have always encouraged it, at least occasionally. This is just a piece of the turbulent adolescent picture.

You may end up inviting your stepson to live with you -- and even your stepdaughter one day -- but you don't want anyone to show up on your doorstep uninvited, especially a stepchild. If he did, he would be controlling the situation and he's done too much of that already. It's much better to grapple with a problem before it becomes a crisis.

You can do that by inviting your stepson for the summer, or a large chunk of it, with some good activities planned -- play or work that is hard and productive and fun. The rest of the time should be spent in a fairly organized way.

He should fit into your schedule, rather than you fitting into his, and at 16, he should do some of the cooking and gardening and home repairs, alongside you and his dad. Even a recalcitrant teen-ager will work if it's expected, if you assign jobs that suit his abilities and if you praise what he does right and ignore much that goes wrong. Success, and recognition for that success, breeds motivation better than anything else.

He'll also learn to compromise -- one of the most important skills of all -- if you lay down rules so the household will run smoothly: a stereo at normal levels, a reasonably clean room, a sensible curfew -- and some sort of family counseling, so the three of you can get to know each other better. He may refuse to go along with you on some or all of these demands, but not for long. A houseguest -- even a stepson -- is in no position to negotiate, or he won't be a houseguest any more.

If the boy needs a more structured home, and if he's willing to live by your rules, he deserves the chance. Somehow, somewhere, he's got to learn the joy of doing before he goes out into the world. And as for your own family plans, consider the timing. Even if you got pregnant now, your stepson would be leaving before the baby was 2.

As threatening as it may be to you and your marriage, you have to support your husband's need to care for his children and help him find what's best for them. They're part of his package.

This may not fit into the storybook image of marriage, but a rich marriage seldom does.