Agree with the other posters. Siblings already have enough other issues to deal with. Sisters sharing the same man is just a receipt for disaster on a colossal level.
I understand where you wife is coming from. I also grew up in a very strict Christian background. This Christmas was hard, both my brother and I made the mistake of telling my mother some of our view on Christianity, Christians in general and the Bible. While I still identify as a Christian, my beliefs no longer follow mainstream teachings. It would have been so much easier to just let her continue believing that we shared her views, but her intolerance just sets me off. As my brother said, it is hard to be tolerant of the intolerant.