[Kevin]

Everything above, I agree with 100%. Several of my fraternity brothers' wives dated someone else in our chapter before settling down with Mr. Right. If it's really over, it's really over. If he's happy, be happy for him. If your sister is happy, be happy for her.

As far as your chapter "not having any bylaws," make sure to keep it that way. To be hauling sisters to standards due to private personal life stuff would just be ridiculous. It does appear that they've at least tried to be somewhat discrete and aren't parading it in front of you. That may very well even be out of respect for your feelings.