Hey guys thanks for the replies....I do have to add that she did for a period of time live with my boyfriend and his family while they still lived in California....not sure if that really makes a difference though.
I had never met his brother so I didn't see it as my place to come along, but I still don't see it as her place either.
She had chased him down for a couple of years trying to start a relationship with him before he finally said yes...he admitted that he had never really wanted to start one, and that by the time she got pregnant he had already been planning on breaking up with her...their relationship was very rocky and the break up came as a sudden surprise to her...she was always very controlling with him and I feel that she does want to be with him still...he admitted to me that before he told her about US she was flirting with him a lot but once he told her about us she was pretty cool about it....and said she trusts me to be around their son...so I just feel guilty like I may be reading too much into it?