It is really unsettling to know that toxic people like this do live out there. In some sense you can only feel pity for them, this girl must have gotten it from somewhere and it sounds like the parents are the reason for it.
This is a really devastating situation, which is all the more difficult with the responsibility of raising your other two children. No one under any circumstances is deserving of abuse, especially from grown adults with kids of their own. I really hope that despite everything, you're still managing to take care of yourself as best as you can. It's never easy having to be the rock for everyone all the time when there's a heavy weight in your heart.
You're right, this isn't a normal situation. I think unfortunately, people have to go down some awful paths to find themselves in order to come out of it even stronger. Manipulators seem to prey on the insecurities of other people to get their way, but they can't eat away at someone forever. Eventually they run out of things to prey on or they'll take their power too far, in which case her true colours will come out and he'll come to his senses.
It's more than understandable to feel helpless and wish you had more control to change his mind. But I think ultimately, there's a lot of peace that can be had in knowing that there's always an open door when he comes back to the place where he belongs. The day will come when he will come knocking.
Sending a lot of warmth your way.