TMI ... Your best advice on my current situation

Linda
Basically , my BFs step sister was a bitch to me since day 1 , and even then I'd always just be nice anyway 🙏
And it got to the point where she'd say she hates my voice , the fact that I'm happy with/about my pregnancy and she was talking down about me to my BF telling him all the reasons why he should leave
And he'd protect her , I'd noticed by just the little things he'd say about her...
So much that I just felt so uncomfortable and alone
Cuz going into a family , my only concern is really just getting along with the new family 
But little did I know that they had actually been having sex for 3 years , and the last time they did was while I was already with him for a month 💔
His dad and stepmom don't know all the details
And I don't want her around my baby
He already 'dropped' her , but idk how I'm gonna explain to his dad and step mom why I don't want her near me or my baby 
Keep in mind I am 24 weeks pregnant. I'm not leaving him , because that would've selfish of me to do ... But I do need some advice 😓