Wow! That really is horrible. I would break it off and tell him why. One he cheated on you in your own house. With his sister of all people. I would not give the option of counseling. If anything he needs to see a doctor and quick.
I'm so sorry this happened to you but it's good you found out. Your child doesn't need to know who it's father is. Somethings are better left unknown and this is one of them. If he can seduce his own sister what will happen to your child. Run girl, run.
Oh my. If he is sexual with his sister, I think you should break it off. That's not just cheating which is horrible enough but with his sister. Gross. That is something j doubt you'll ever get over. And it's likely not thd hurry time. Living with you, impregnating you, was not enough for him to give this up. You can't trust him. I do wish you weren't pregnant. This is Such weird situation to bring z child into. Ugh. I wish you the best but your baby's father is s chest and an incestual cheater. That's unacceptable. Good luck