Boy it’s tough. Sorry you are having these issues with your sister. Does she live far? If not maybe ask to meet face to face? Write a letter? Or perhaps just let it be, which must’ve painful, but maybe time will heal? Maybe it’s too fresh? I’d not involve spouses though. If I had a fall out with my brother, I’d be taken aback if my sister in law was the one trying to talk to me about making up and telling me how he is suffering.

I’d try to make it one in one with her, no other family meddling. But maybe wait a bit longer. Do you think it’s entirely her fault or both? If both, does apologizing help? You said she doesn’t call you, do you try to call?