From the situation your explaining its sounds like it would work out fine. Though In most cases I am highly against it since it can cause big rifts. My hubby was dating a girl, they lived together. Well they were having issues and his little brother had got out of jail and needed a place to stay. Hubby and the girl split, though still living together. Well his brother made a move hubby was like whatever and let them, never telling his bro how he felt. (I saw him crying and really upset over it at his sisters house, who I was friends with, long before we got together) they ended up together got married. Hubby slept with her again after his brother went back to jail (can anyone tell yet why I really don't care for my bro in law) sometime after me and hubby got married she decided she should tell her soon to be divorced husband that her and hubby had slept together. Bro in law was furious and was horrible to hubby for a while. Me and bro in law had a chat about it and I told him to look at it another way he was seeing it as a betrayal by his brother, being outside the situation I saw it as he was getting revenge for his brother taking his sort of girlfriend. Its calmed down now but there are still issues there!! Hubby is still nervous I would go with his brother (OMG GROSS) and even though the other two got divorced we still get stuck seeing them together (they have 3 kids) I even had a chat at one point with her, she asked me why I didn't like her. I told her i don't really know you but what you did was morally and ethically disgusting to me. Brothers, friends exboyfriends of friends all off limits in my book. Good luck with whatever you decide to do!!!