aww Lostmom, this sounds like a nightmare. yes you need to get out. if you can't do it now, start planning, check with county services about shared living with other women.
the whole thing is totally dysfunctional. sounds pretty obvious that he had sex with your niece, and he should probably be charged with it. i don't want to set you off, just saying what looks like to me. and sounds like he is having sex with other women, whatever the reason it doesn't really matter. if you want a committed relationship, you deserve that.
i think it's clear he's fooling around, and he's hinting at coming clean about it, so he admits to smaller things. he probably won't ever really admit what he's done, especially the illegal nature of the niece, who should've had him watching out for her, not having sex with her.
i'm so sorry you're going through this. last thing you need with all the stress with the kids already. but you must start planning to get out. maybe you would be single for awhile, but you deserve to find a man as strong and caring as you!!!!
be as strong as you can, please don't hurt yourself, you are very loved. pm me if you want. xoxo