The answer from the hip is who do you have to sleep with, but in actuality I think that you need to weigh the dispute carefully and try to remain neutral.
I don't have a sister, but my brother and my ex-wife were in a bitter fight over his intended marriage and I sided with my spouse (simply because I had to live daily with her). It caused a major rift between my brother and me and in the end, I learned she was wrong and lied to me. Of course she lied to me about lots of other things too which is why she is my ex. It was a lousy business and looking back at it, I think it's best to remain neutral and I would have done better to do that if I had to do it again.