Hi, Carolyn: I have suspected since Day One of meeting my husband's family that my sister-in-law did not care for me, but I get along with the others just fine. She is gushing when she sees us, but will talk about anyone who leaves the room. She gossips about everyone and says things about others I know not to be true.

My brother-in-law's wife recently shared a boatload of gripes this sister-in-law has about me, including horrible things she said about my children. The person who told me what she said about my children was upset and thought I should know, especially since she said these things as she was holding court at a family event we did not attend.

Preferring not to start drama for my husband's sake, I have gone with the flow for many years. The comments about my kids hit a nerve, though, and I want either not to see her again or to let her know what I heard. Advice?

— H.

H.: You’re probably right that she doesn’t like you, but that’s also beside the more important point: that she treats everyone abysmally, not just you or your kids. And apparently no one is calling her on it.

As someone who often discusses the value of peacekeeping in extended-family situations like this one, I hope you’ll trust me when I say this peace is not worth keeping. It’s the peace of enabling, appeasement and rot.

The wife who tipped you off did sort-of the right thing in getting upset and calling the meanness to your attention. The entirely right thing, though, would have been to summon the strength to stand up in the moment, before the whole court, to say: “Stop saying these things about X and her children.”

So that’s what I’m going to advise you to do about this, above all. You know the sister-in-law trashes people behind their backs, so you’ve witnessed it firsthand, yes? Bystanders have enormous power to call out cruelty, to identify it as unacceptable, to stop it — when they have the courage to. Your impulse now is to stop the abuse directed at you and your children, rightly, but stopping the abuse on others’ behalf as well as your own is the side the angels are on.

For your husband’s sake, talk to him about what you’ve witnessed and what you were recently told. Explain why you have let it go all this time, and why you won’t let it go anymore. If he asks to be the one to take it on, then allow him that, of course — his family. But if he asks you to keep letting things go, then please hold firm. Say you’ll bend on how you do it, but not on what needs to be done.

Hi, Carolyn: My wife is from the other side of the country, where all of her family still live. Her family is extremely tightknit — she's the ONLY member who lives out of state and one of few outside a two-town radius. I applaud her in her ambitions and willingness to leave her home state to find a great job.

In the four years she's been here, her siblings have come out to visit her on a few occasions, but never her mom, despite my wife asking her to. I know it's not an easy trip, but her mom has gone on five-plus-hour plane rides before, so this comes across to me as rude.

My wife is very close with her mom so I see this as an even bigger slight. Is there something that can get her mom out here? Something my wife can say? Or should I pass this off as a hopeless cause?

— Anonymous

Anonymous: I love that you’re backing your wife.

But I doubt you’ll help if your way of backing her is to start banging your head against the same wall she is.

Instead, be the one with the ice pack. Listen to your wife, make sympathetic sounds, say you’re sorry her mom won’t come. Say out loud that you don’t understand why and don’t think it’s right. Ask her directly if she’d like you to get involved or if she’d prefer that you just listen.

The impulse to fix her relationship for her is natural and understandable, but it’s her mom, her relationship, hers alone to fix. So it’s your impulse to tamp down.

If your wife does invite you to offer ideas and talk about what’s going on, then please consider that her family unity is one “2,” her breaking out is the other “2,” and the mother’s absence equals the “4”: If a daughter “very close with” her mom broke hearts and precedents to execute an unheard-of departure from the nest, then the mom might be absent in passive-aggressive protest. Not to say that’s true — or mature, if true. Just possible.

Fingers crossed this is as bad as things get, and the mother drops her boycott as soon as pandemic conditions permit.

Write to Carolyn Hax at tellme@washpost.com. Get her column delivered to your inbox each morning at wapo.st/haxpost.