Q.It was clear that something had been bothering my teenage daughter for some time, that she felt bad about herself and that she needed to tell someone about it. And so she did.

My daughter told me that her older brother sexually abused her eight years ago.

Initially she said it was a single incident, but she later said that it had happened "a few times," that it was "no big deal" and "in the past" and that she didn't want to talk about it with me, with her brother or even with the psychiatrist who treats her depression with talk therapy as well as medication.

I talked with my son anyway -- without my daughter's knowledge -- because I needed to hear the full story and to hear it from his perspective.

This was extremely painful for both of us. Although he didn't want to discuss it at all, he did say that he had touched his sister inappropriately and that there was partial nudity, but he said there was no sexual intercourse. He also said that he will never forgive himself for what he did to her and that he has tried to put the past behind him, but I suspect he is still haunted by it.

Since my son and daughter don't want to discuss these incidents with each other, or with a therapist, how do I help them heal? How do we get beyond this? I feel horrible about what my son has done, horrible about my daughter's pain and her long silence, and horrible about myself for not protecting her better.

A.Your son may not want to talk to his sister about these incidents, but sometimes we have to do what we don't want to do.

It's time, and past time, for him to apologize to her directly, so that they can begin to build a healthy relationship again.

Your son should tell his sister that he is truly sorry, and that he's been ashamed of his behavior all those years, but he doesn't have to tell her that you have talked to him about it or that you told him to apologize to her. It's what he's been wanting to do anyway and it's what she's been waiting to hear. If more people said they were sorry, we would have many fewer divorces, family feuds and nasty lawsuits -- and a much happier world.

When your son tells his sister that he's sorry, he will be bringing the abuse into the open, which will be good for both of them. The more they suppress their moldy memories or push them into the past, the more it's likely to warp their lives. It takes sunshine and sympathy to make mistakes go away.

Although it's only natural that you feel wretched and deeply saddened by your discovery, please remember that you are not alone. Whether parents call it "fooling around" or sexual abuse, it's more common than you think, especially if a brother and sister are home alone and the boy is in his young teens. Sexual desires can be overwhelming when his hormones first kick in and he may find it very hard to resist any female -- even his little sister.

This situation occurs even more often in stepfamilies, if the stepsiblings are unsupervised and they haven't lived together long enough to bond.

That's why many working parents and stepparents try to have a grandma or a college student around the house after school, ostensibly to start supper, tutor one of the children or run the vacuum but mostly to keep risky business away from their door.

Although you are deeply upset, your children are still your children, no matter what they do, and you can't stop loving them. The best you can do is forgive your son for his trespasses, shower your daughter with encouragement and make sure she takes her meds. A trauma such as the one she experienced can dissipate some of the serotonin in the brain and cause a clinical depression. If untreated it could come back again and again.

And while you're forgiving your son and comforting your daughter, please forgive yourself, too.

Children get into all sorts of scrapes and do all sorts of dumb things while they are growing up and parents can't protect them every time. In fact, they never find out about half of them.

Questions? Send them to advice@margueritekelly.com or to Box 15310, Washington, D.C. 20003.