AITA for telling my MIL I didn’t want to have a one-on-one coffee with my SIL?

NAH. I think perhaps you didn't realize how much this meant to your MIL or how serious she was about it. You responded as if it was a flippant half joke from her. But it seems like she meant it deeply and may have put a lot of thought into the request before calling you. And then you just blew her off.

While I don't think you were TA for your reaction, I do think you would be TA if you don't change your mind. It's just a coffee. Your SIL will probably act no different after, and you can still avoid her when possible, but at least you gave your MIL the gift of trying.

If SIL was mean or abusive to you, then I'd say otherwise. But from your post, your only issue with her is that she's self centered. That's not so terrible and while I understand avoiding her drama, there's no reason to avoid her completely and not give your MIL this simple gift of a gesture.

/r/AmItheAsshole Thread