NTA

You will basically have to take care of him, his kids, and his baby mama, and not just financially, I'm willing to bet that they will expect you to take care of the kids too since "the mom has cancer and can't take care of them".

I just want to ask you, why are you dating this man? Why choose to deal with all this drama when you could just have fun and date good men that spoil you? You are only 24... It's such a waste of time. Unless you're 30/40+ then you shouldn't be dating men with kids. Most men wouldn't date you if you had kids either so why are you choosing to be this miserable? A man that has kids = lots of baggage and drama. I'd say break up with him, move out and move on with your life, I hope you have enough self esteem to not stay with him and leave.