The title caught my eye.

I guess it boils down to - do they {family} make more negatives in your life or more positives??

If the interactions are mostly negative then distancing yourself is a good choice.

Wendy-
But if your sister is basically a nice person -then maybe send another card with a "Best Wishes" and a "I just was told of your wedding" "thinking of you" that sort of thing and see if she replies or not???

but only you know if that is worth a try or not.