Q: Last year my 35-year-old sister told me about a terrible thing that happened when she was home on a visit five years ago. She awoke briefly from an alcoholic blackout to find our father in her bed. She's not sure if she encouraged this get-together or if she was raped, but the next morning he went to her room and apologized.

This incestuous incident prompted her to get treatment for alcoholism, which was successful, but she has other problems. She is the oldest of 11 children, has never really liked our mother, blaming her for all the problems in our parents' marriage and in her own life. She now thinks if she tells my mother what happened, the two of them could perhaps be friends.

I persuaded her not to tell my mother anything, at least not yet, because I didn't think it would help their friendship and the consequences would only destroy our family.

My sister told me about it because she saw a closeness developing between my father and me and she didn't want me to suffer, too.

Of course many other things have been going on throughout the years, including an affair my father had 10 years ago. I didn't tell my mother when I found out about it but I did speak to my father. He denied it (even though it later proved true) and for the next few months his actions and attitude toward me were so cold I had to leave home.

My sister, who then thought of herself as my father's confidante, also found out about this affair but was supportive to both my father and his mistress.

He's always pretended to have such high moral standards, expecting all of us to live by them and getting so angry when anyone deviated. He is so critical of the women in the family, so holier-than-thou!

Recently, I decided to tell him that I knew about him and my sister, because I can't live alone with this information and the deep hurt I feel, even with professional help. Should I send him the letter I've enclosed or confront him in person?

A: You definitely don't want to send your father a letter about this, out of love and respect for your mother. There's no such thing as a secret, and when it's committed to paper, you never know when it might be found or by whom. On some level your mother knows already, but she sounds like she probably has all she can handle. Nor will she embrace your sister for telling her about it. Bearers of bad tidings are seldom thanked.

As much as your father's behavior has skewed the family, and as painful as this new revelation has been to you, it's your sister who has the most problems to resolve and the most confronting to do.

In a sense, she's started the process, but she's still aiming her anger at your mother by talking to you as your mother's main defender. There's no point in either of you carrying on the arguments of your parents. You are your own people, who have a common problem.

Since you feel it so acutely, you're the one who should seek help with a first-rate family therapist who will then ask other members of your family to join you for some sessions.

This therapy would focus on inappropriate behavior and relationships that have gone awry. The incest and the infidelity are symptoms. Although you would be the main client, the therapist would help all of you learn how to deal with each other honestly. This is the only way a family really works.

Since incest usually is the result of generations of trouble, it's important that your father get into this therapy. Even if he never initiates incest again, he still needs help to deal with his past. Without it, his integrity will be under constant attack, and he will continue to attack the honor of others.

If he does join you in therapy, you'll have to be as honest and direct with him as possible and as unemotional. Hysterics would only make it less productive.

Tell him the things you had written in your proposed letter -- how hurt and disappointed you are and how you want to salvage whatever love and trust for him you can. This won't be easy, but he surely gave the family some things that deserve your appreciation. It's harder for the flawed to do their best than for those who find it fairly easy to be good.

Your sister probably will come to some sessions with your father, and you might even decide to ask your mother, for honesty -- and professional help -- are the only things that will make their marriage work.

If your father won't go to therapy for your sake, he may go for his own. Tell him face-to-face what you know and threaten to tell the rest of the family if he doesn't go or if he tries to freeze you out of the family again.

You'll also have to tell him that you and your sister will watch out for younger sisters or grandchildren at home. This is a tough stand, which you shouldn't embark upon unless you think you can handle the consequences. They have a way of turning scenarios upside down.

If no one will agree to family therapy, you and your sister should continue on your own and expect to be in it for quite a while. It takes longer to change when you change alone but at least you'll be breaking the cycle.