Cheating husband, may lose custody of my kids

I have 3 kids with my husband and I'm pregnant with the 4th. I found out my husband had been cheating on me with multiple women for a year. I told him I want a divorce and I'm taking full custody. I told him I'm taking him for everything. I of course was still gonna let him see the kids though. I kicked him out and that was that. Well he came back and said he wanted to talk. I said I had nothing to say to him and he said well okay just know I'm taking full custody of the kids. I said "No you're not". He looks me in the eyes and goes "Honey. You're a registered sex offender for child pornography. It's not gonna be that hard to fight." Before anybody calls me disgusting, let me explain. When I was 18 I was dating a 17 year old. We exchanged nudes back and forth. I didn't completely understand the law. It was legal for us to have sex so I figured there was nothing wrong with us exchanging nudes. I was wrong. You can not have nudes of a minor or send your own nudes if you are a minor. It doesn't even matter if you are BOTH minors. It's illegal for minors to sext. It was found out. We both had to register. Me for having his nudes and him for distributing his nudes... If I would have understood the law better I wouldn't have done that. My sister has worked in family court before so I asked if they would take the actual story if what happened into consideration. She told me she's gonna be honest with me. They don't care why you're on the list, what matters is you're on the list so my ex could have a good chance of getting full custody but I would still get visitation. I'm panicking rn. She told me "Look. I know you don't want to, but you might have to put your pride aside and go apologize to him for saying you'll take him for everything. Say you'll take what's yours and he can take what's his and go for split custody. Keep it as civil as possible because he has a good chance of getting full custody if he fights for it." I really don't want to do split custody. I wanted full and allow him to visit but it seems I may have to settle...