How do I get over the fact that my sister has betrayed me by cheating with a (now ex) boyfriend behind my back?

Bit of a back story, my family is very close and we all are always together hanging out so I didn't think twice about my sister and my boyfriend at the time getting along with each other. I didn't realize how weird it was that they'd text each other, mostly because my ex and I were constantly together — basically inseparable. I believe it all started when I went back to school because once I'd get home, she'd be walking out the door.. of my house.. wearing her PJ's. This has happened multiple times but I didn't think anything of it because she was going through a tough relationship and would stay at my house sometimes. Anyway, we ended up breaking up due to other reasons and he made a comment on Facebook to some females that I don't get along with, basically joking about our break up and that he didn't care because he slept with my sister. Obviously I was extremely hurt by that and I started realizing all of the times that they'd flirt right in front of me and I was just too stupid to even notice it.

Now, I didn't notice on how much it bothers me being that it has been 6/7 years since this incident and I'm in a new relationship (for 5 years) we have a 3 year old and I'm currently pregnant, I didn't notice how much it bothers me if my sister comes around my man or if they even conversate together I become extremely insecure because I see how she flirts and my man is just fucking blind like do you know that in her mind you're sending her signals?! I don't know how to move on or if I can ever trust my sister alone with my man. I just know how she is and she will wear skimpy clothes around my man or tease me in front of my man and put me down about just about anything (she thinks she's better than everyone) she's even called me fat, I'm 7 months pregnant! Ugh