Ah, I see now.
Older adopted children (we tried this twice) oftehn have attachment disorder due to the neglect/abuse before they came to us. This is as bad as having antisocial personality disorder. We had to undo one of our adoptions because the child molested my younger ones and not just once. We didn't know it. YOU don't know if this child is molesting your younger kids. No, they would NOT tell you as they fear their abuser, who usuallyl says that if a child tells, he will kill all of you and the young ones believe in his power over you.After all, to them he has God-like (or Satan-like) power to be able to do what he does to THEM...why not you? Our child told them he'd burn the house down with everyone in it, then started small fires to prove his point. He held knives to their throats to make them perp. We did not know it. I still feel guilty about that. I always will, but he was so sneaky and the young ones were only just 4 and 6...he was thirteen at the end of it and they believed his threats and did not tell and we found out by accident, after it had been happening for a long time. It actually ended when he killed our dog and I knew it was him because nobody else was home and the dog was choked with his own leash. So bad stuff happened for a long time before we found out and called social services to come and get him...forever. We haven't seen him again and never want to.
He acted like a loving angel to all adults so he fooled social workers, psychiatrists, other foster parents and us. Cunning.
I would not get further involved with the father, no matter how much you love the father. If you are married, I'd leave with t the other children until troubled child is in residential care where he can't hurt the children anymore. After what we went through, I don't believe in taking chances when a child is dangerous. They can also be sneakier than you could ever dream. They trust nobody but themselves and it is hard to treat attachment disorders and his sounds serious, unless he is truly a sociopath in which case they have not yet found a way to change these sort of problems.
Hugs...I'm so sorry.