I was at odds over an issue- with my Brother, several years ago. It took us a while to work things out. Both of us did our share of bitching about the other..... guess we finally realized what a waste of time it is. However, the time apart did allow me time to reflect and learn some things.
I hope that your sisters BF will eventually accept, deal with or otherwise come to terms with all of this. I hope that it will result in a stronger relationship for all of you.
No fun to have a divided family....... But perhaps he will use his time apart to reflect and learn.
Principles and ethics are two things that so often- get disregarded for the wrong reasons. I don't envy the pressure all of your decisions brought- but to preserve the principle and ethics of this situation was the right thing to do.