Cheating.. questions for ladies?

Hi, so my fiance and i got together when I was about 2 months pregnant well not even with my ex bf's baby, I didn't stay with him because he was a very very bad person and I didn't want him to be a part of our child's life, and then I met my fiance who I have been with for 3 years, and he was with a girl about 9 1/2 years ago and they have a 9 year old daughter together, but when she was 7 months pregnant she said he d her (which he didn't) and then he said he would have her charged with slander for lying, and then all of a sudden her and her family packed up with out telling him and moved like very far away. He never got to meet his daughter or ever talk to her just seen pictures and never talked to the girl he was with again. His mom sends his daughter gifts for her birthday and special occasions and stuff like that, and so do we ( me and him). But so about 9 months ago we were doing fine, had our own home on the go, him and my daughter love each other very much even though he's not biologically his he excepts her as she was, and then all of a sudden this '' girl'' he had the baby with moved back here after like 9 1/2 years, and of course she's like model pretty and I'm average .. so of course here i think he's going to cheat on me with his old fame!, but so anyways a month after she was here i found out i was pregnant, so we were happy, but i just can't get her out of the way, he say's he hates her for saying that stuff and leaving him and taking the unborn baby with her, but it's hard to believe that, so anyways i told his sister about this and she agreed with me, she thought she was a ***** too, and she was best friends with the girl's sister, so it was hard for her because she lost her best friend too. but about 6 months well 7 months after the girl was here her sister's boyfriend beat her and she needed a place to stay so she moved in with my bf's sister and mother and now his sister is best friends with the girl he has a kid with and her old best friend too. This is really hard on me because now his sister doesn't even talk to me, she just bothers with the other girls now, and is very stuck up now, why is she doing this to me, i just want to be a part of their family and now they are leaving me out for a girl who said that there brother d them when he didn't... I feel so alone, and i accuse my bf everyday pretty much of cheating on me with this girl because ever since her sisters been here he's known a little too much about this girl he use to be with, and she's honestly been sleeping with every one in town, and she has kids, this isn't right, what do i do, do you think he would cheat on me with her and not tell me, or what?