Q: What should I do with a close relative who throws temper tantrums?

She is my dear brother's wife, 40, and the mother of two really quite well-mannered teen-agers. She is a dear in her own right when she wants to be, but her uncertain temper makes her a social liability -- similar to, but not socially acceptable as, an epileptic who is subject to grand mal seizures just as I hand out the champagne in my best crystal.

The latest scene was just dreadful. Her whole family was visiting me with plans for supper out and an evening downtown -- she, he, the two adolescents and a friend of my nephew's. When my nephew and his companion lollygagged back to my house six hours after their arrival in town, my sister-in-law went into her fit, but not at the teen-agers. Instead, she splattered invective at my brother and around my dining room for 45 minutes. I left the room just as she got fired up, hoping she would regain her composure before I returned, but I finally had to come out of the basement and acknowledge the scene.

She spoiled the party. The evening resulted in my brother's returning to work as an escape, her sulking over a book at my house because she did not feel like going out, and my taking my niece, my nephew and his friend to dinner downtown. The children were subdued, at best. I was trying to salvage something of the evening, with no success, mostly because we had planned the evening to be an "easy family party," especially to benefit the children. You are ever so right about "easy family outings" -- they are much harder to achieve than full-scale formality.

I am still irked at her. It is unlikely that an apology will be forthcoming -- she believes the Southern California claptrap that rudeness is enlightening. I know I cannot tell her that she was rude, boorish, ill-mannered and a slatternly fishwife in my house, for heaven's sake.

It was tacky of my brother to apologize for her behavior, I thought. It also was tacky of the two of them to expect me to enter the fracas.

Do you have any suggestions for how I can behave myself in the numerous family parties (my sister and her family and my parents live in the same metropolitan area) that are scheduled? May I disown relatives discriminately? Can I pretend that I had moved to Europe? Or that I'm out of town for the next six months? Can I pretend I died?

A: Miss Manners has never cared for the widespread use of the medical model in describing disgusting behavior. In her opinion, your situation is in no way comparable to that of someone with a relative subject to epileptic seizures. All decent people accept manifestations of physical illness as a fact of life, in which embarrassment is inappropriate in both the ill person and the witnesses, as no social transgression has been committed.

Miss Manners will now pause for a chorus of protests my those who consider themselves decent, and Miss Manners indecent, because they believe your sister-in-law's behavior to be evidence of emotional illness, equally worthy of being treated with tolerance.

Very well. Those people will have to explain, then, your sister-in-law's philosophical justification for her rudeness, which indicates to Miss Manners that it is nothing more nor less than what you call it -- an adult temper tantrum.

But this is a side issue, because the social solution here involves treating the behavior as an illness -- hypocritically, in Miss Manners' case, but effectively. Say to your brother, "I'm afraid dear Marietta is not at all well, and we simply cannot expose her to the criticism of those who don't understand that she can't help behaving the way she does. I know I'm helpless in knowing the best way to treat her. Perhaps we better not subject her to these gatherings until she is herself again, and able to handle them."