Should I go to my neice's wedding?

If you want to go to the wedding, you have been invited. RSVP to warn them. If you don't really want to go and deal with the whole crazy family cloak and dagger stuff, RSVP that you aren't coming but be magnanimous enough to send your niece (whom, I'm uming, isn't being difficult with you) a nice wedding gift. If you go, enjoy yourself but don't expect a lot of family warmth and love, go just for your niece. If things start to get weird or negative with your sister and her spouse, if they start embarring you or start making you feel too uncomfortable, just leave--give your niece and her husband all your love and best wishes before and leave. Keep your distance from yoru sister and her life since you aren't welcome and it doesnt' sound like a place that you'd want to be anyway. Your sister obviously has problems. Get on with your life and make it seem enviable to your sister so later, when she finds out, she may reconsider the "you can't call us" rule--not that it matters. Find new "sisters" to offer moral support and sisterly love--sometimes the best sisters are the ones you aren't blood related to.