Dating this girl a few years back and I became really good friends with her brother just because he appreciated my help during a difficult time for their family (their father had abruptly passed away).
2 years into the relationship the brother contacts me and asks to meet up for coffee and a serious conversation.
Apparently HE caught his own sister with another guy (mutual friend of ours) and this shit had been going on for like a year.
As soon as he found it, I was told.
Broke up with her. Still friends with her brother. Go figure.