I was in a relationship with my good friend of 10 years brother a few years ago. She was always supportive of the relationship and it was her who actually set us up. We split up and he ended up starting a new relationship with a mutual friend of mine and his sister. Me and his new girlfriend fell out as I wasn’t comfortable being friends with her and we left it at that. They split up a year ago and we made up, however we both knew this was more of a civil friendship rather than true friends due to the issues in the past. Anyway, for the past few months I have been having a fling with ‘our’ ex but I recently found out that she has also slept with him recently also. My original friend and his other ex found out about our fling and have fallen out with me. I haven’t spoken to them for months because they said I was sly for going behind their backs to sleep with him even though the other friend has done the same thing. Am I really in the wrong here or is she wrong too? And how can I make up with my friend of 10 years (his sister) as she was one of my best friends and I’m very surprised about how upset she was over this?