Question: My sister "Rhonda" is about to give birth to her third child. I have a 1-year-old. During most of this pregnancy, my sister has acted almost as if she hates me. She is increasingly argumentative, taking offense over seemingly innocuous conversation. It leaves me feeling crazy.

I have repeatedly asked my mom and my sister-in-law if they know why she is being so hostile with me, and I kept getting the, "Well, her hormones might be out of whack," etc. I continued to insist it must be something else, because I am the only person she treats this way.

My sister-in-law finally confessed that Rhonda is angry with me because she believes I broke her rocking chair. I have no recollection of breaking the chair, unless I did it sleepwalking. I do not know why she thinks it was me, and not my husband, whom she continues to regard fondly. My husband does not recall the chair breaking, either.

My mother and sister-in-law told me I am not allowed to ask Rhonda about the incident or I will be breaking Rhonda's confidence. So I can't really apologize, or make it right, or buy her a new chair. What is this?

Answer: It's apparently irrational, possibly hormonal, and definitely a huge fail by your mother and sister-in-law.

When someone chooses to mistreat another for a specific, undeclared, batpoop-crazy-petty reason, as Rhonda is mistreating you, informed bystanders have a duty to step in, especially when they're family. "Rhonda, you are being openly unkind to your sister without even telling her why, much less giving her a chance to make amends. Either you talk to her, or I will. You have a week."

It felt good just typing that. There are plenty of confidences we're bound to keep, but allowing nasty, secret grudges to poison a family is not part of any duty to remain discreet. Witnesses have significant power in these situations that they often don't use for fear of meddling (or fear of Rhondas). There has always been an exception to no-meddling rules in the event of cruelty, though, and shutting someone out for unexplained grievances is just that, cruel.

So enlist your mother's help . . . a month or four after the baby is born, since a little oxytocin might put Rhonda back on the rails.

This isn't to say that only rainbows await if your mother intercedes. Rhonda could easily turn on your defenders - and blame you for that, too. Or she could acquiesce on the Great Rocking Chair Calamity of 2014 only to turn on you for something else. People are less likely to second-guess passionate feelings than they are to justify them in arrears. If she's got it in for you, expect her to keep it.

Still - when a family stands up for civility within its ranks, that's its only real chance to stay on its rocker for good.

Chat with Carolyn Hax online at noon Fridays at www.washingtonpost.com.