I was in the exact same situation.
I was seeing a guy, fell pregnant and found out he was actually engaged to someone else, had a kid with her and also had 7 other kids to 7 other women, all of who he had nothing to do with.
I was 20 at the time, very young and very hurt. All hell broke loose on facebook, texts etc etc. I had him coming back to me telling me he loved me and was going to leave her etc. He was saying to her that he doesn't know who I am, has never met me, I'm making it up etc.
For some reason, she believed him and they stayed together.
I remember being heartbroken and so upset that he was with her and not me. He has seen my DD twice in 4 years.
Still to this day, I get hassle from her and her family. I moved away to get away from it all and im happier now but whenever I visit my old hometown it all gets brought up again. They have split up now and he takes nothing to do with any of his kids.
From experience, the only advice I would give you is to keep your head down and don't rise to any arguments or nasty comments that will be made. Ignore them and get on with your life. He will never ever change and even if he does decide to be with you, you will always wonder who else he is seeing and she will never leave you alone.
What goes around comes around and he will be left with nothing at all.
So will she hopefully.