May I assume the younger brother and your sister are twins? How old?
As for the rest, I cannot imagine what good would come out of contact at this juncture. Your kids don't need to be exposed to this stuff.
Cutting ties sounds so permanent. Why not just take it year to year? Maybe something will change and pigs will fly
Could you meet the twins somewhere for lunch and do something fun for all of you? And yeah, the rest will cluck-cluck about it. Does it really matter so long as you protect your children from chaos?