Girlfriend always talking about with the wrong people?

Ok hopefully someone can shed some light on this subject for me. I started "dating" this lady about 7 months ago it was an instant attraction but I believe it was for all the wrong reasons. I moved to a different state where I knew nobody and she also moved to this state knowing nobody but her boyfriend and his family. She's 13 years older than me and her boyfriend is 13 years older than her and they're not intimate in any way shape or form. She was a very bold lady because we would go to the movies with her boyfriend and go bowling and do things together with him but he never knew we were having . Well what happened was this lady went to her boyfriends brother and told him that we were having she told him everything about us even the most intimate details. At first his niece was telling me that he didn't like the idea that we were sleeping together but I never heard her explain it to me that way it was always everything was alright I even met the guy one day and he made it seem like everything was alright with him. My point is this though I think she's sleeping with her boyfriends brother because she always comes back home and tells me about how they're always talking about and touching on one another and I mean I'm 26 I know older people have their ways and in some instances it is innocent. However if she had to keep the fact that she was sleeping with me away from her boyfriend there's no way his brother is asking her is she still getting any around his brother. He's married but his wife is like 12 years older than him and this woman that I was intimate with is a very repetitive woman now she's going over there early in the morning and she's in constant contact with him she even calls him on his job and all. I just believe from what I've seen that it's a possibly that they are sleeping together. I hope someone can help me sled light on it am I just young and insecure or did I have a reason to be concerned. I say did I have a reason to be concerned because I'm not dealing with her no more.