Broken relationship with sister

I would make an effort and go. Your sister won't be able to rant if you are all there. Safety in numbers. She is probably looking for your approval and has a strange way of showing it but younger siblings often put their elders into the position the parents had, IYSWIM

It's a fine line to walk but will be worth it. She may have moved on a bit and it's got to be better than simmering hatred.
From Robert Burns' Tam O'Shanter :
Gathering her brows like gathering storm,
Nursing her wrath to keep it warm.

I would meet up , chat to friend, chat normally etc. If your sister asks anything provocative just say I'd rather not discuss that and change the subject. Have some subject matter in mind ready. If she starts to kick off say that you dont want to spoil the get together for everyone and as your presence appears to be a problem you are leaving.

I think that is what I would do in this situation

What we actually argue about is often not the real reason we fall out. From experience I would say there are many many buried issues that bubble to the surface

I would have said to leave it, and just get on with your own lives. The only people I would ‘ pursue’, are children and grandchildren. However, since this family member wants to meet, it may be a last chance. Does this person know of the animosity? Could she be trying to fix things? I would go, but make it the last time. Life is too short for all this aggravation. Good luck?

What a dreadful situation to be in Juel. After reading the part about your brothers comments I can't help thinking that there's certainly some underlying issues in your sisters life and her coping mechaninism is venting on a subject she feels strongly about but she is simply not accepting other people's points of view and values.

The very fact that she "kicked off" in the cafe does rather indicate insecurity, immaturity and anger surely?

Perhaps meeting up with the mutual family friend might be a way forward. It does rather sound like your sister needs to talk about her anger issues no come to terms with them. Perhaps at some points could suggest counselling?

I hope you mend your relationship, good luck.