Q. My mother-in-law constantly criticizes my husband, who's 51, and our 5-year-old daughter.

She even brings up things my husband did or didn't do in junior high and high school and she criticizes my daughter so much that our 8-year-old son wants to know why Grandma hates his sister.

She has yelled at my daughter for being fussy, for being clingy and for leaving food on her plate. And when Grandma yells, she yells for 15 minutes, then fusses about the horrible things we're doing and from there it deteriorates into Bill Clinton, Democrats, black people, etc.

We tried to mediate, but last Thanksgiving she became extremely hostile and we left the house. Since then we have only sent cards for holidays and birthdays, but she now assaults us with vitriolic letters and sometimes with phone calls.

She says she has the right to express her opinions, but I don't think we have to listen to them.

I am afraid her tirades might hurt my daughter, and yet the kids ask when they will see their grandparents again.

So far, I have told them that it's best to love some people from a distance and that it's acceptable to walk away from bad situations.

I don't care if we ever see my mother-in-law again, but my father-in-law wants us to mend fences. Do I try to do that?

A. Before you make the separation permanent, make a final all-out effort to mend the family fence.

Tell your father-in-law that you're willing to work with him but first his wife must get a physical check-up from a gerontologist, because this specialist is used to the physical and mental ailments that can creep up on folks as they get older. This is especially important if her behavior has been getting worse and worse.

Once she's been checked, you, your husband and your in-laws should meet to set some rules, but do this with a family therapist, so you can get outside advice. You can expect the therapist to suggest that you meet in your mother-in-law's house -- so you can leave if she starts shouting -- or in a restaurant, since she would probably be more restrained in public. You can also expect the therapist to insist on one rule above all: You must be extremely polite to each other, now and forever.

Your mother-in-law has the right to express her opinions, but she doesn't have the right to tell you or your husband what to do, or to berate you for past or present failures, or to denigrate your children. If she breaks these rules, don't engage. Just get up, take the children and leave.

You may have to pull this routine two or three times before she gets the message, but she should improve once she knows she can't see her grandchildren unless she plays by the rules. If that doesn't work, send cards and presents for holidays and birthdays, but return her letters unopened, let your answering machine screen your calls and skip their invitations.

Even if you have to cut off Grandma, you should stay in touch with your father-in-law.

You also should be patient with your husband. He probably won't be as firm with his mother as you, nor will he pull away as completely as you or at the same time. It's very hard to divorce a parent, and only he can decide when he's had enough.

He -- and you -- may learn to handle your feelings better if you read "Toxic Parents" (Bantam, $7.99), by Susan Forward with Craig Buck . It's helped thousands of adults deal with their troubled parents.

Questions may be sent to margukelly@aol.com or to Box 15310, Washington, D.C. 20003.