I don't envy you this situation at all, but the more you share, the more I've of the mind that your sister may not care one way or another if you attend. And if that's the case, then don't go. And don't feel bad about it.

But, yes, do talk to her. And maybe come right out and ask whether she really wants you (or anyone else) there. You may not get a straight answer, but you might be able to read enough between the lines to figure it out.