I was in a similar boat. We broke up for two months (legit broke up). He slept with someone and then came back to me saying how much he missed me and that he wanted to get back together. I dated someone for a week or so while we were broken up but I missed my ex so much that I couldn't bare the thought of getting close to the new guy, let alone having sex with him. Once we got back together I found out about the other girl and I was crushed. Here I was thinking he was just as sad and as lonely as I was when really he was out banging this girl. If I had know about the other girl prior to getting back together I don't know if I would have got back with him. To this day it still bothers me from time to time because I dont want there to be another girl he slept with in between our relationship. Especially when I couldnt bare the thought of doing that to him. Now everything is great and I 100% trust him in our relationship.

With your situation I am going to say that it is probably true what the sister said. Whether you want to bring it up or not is up to you. If it does bother you and you do end up bringing it up then you have to make the choice of letting it go and not letting it bother you in the relationship or if it does bother you way too much then move on.

When I found out about the other girl I would dwell on it all the time and any time I would think about it I would end up bringing it up and it would start an argument. I had to come to the realization that he loves me and wants to be with me and as much as it hurt me I have to let it go and move on.

Hope this helps! Best of luck to you!