One thing you can do is tell your sister to immediately pick up the dog, that it can't stay at your home. It's already bitten twice and you can't control it. Tell her that if she doens't pick up the dog by Monday morning, you'll bring it to the vet. And then follow through.
I'd especially watch my back with your ex-hubby. I've seen those get quite ugly, so in your circumstances, unless you're commited to really trying to retrain this dog, just get rid of him.
But I think talking to those at the rescue would be of benefit to you. Have you called them yet? Rescues sometimes work over the weekend. I would tell them the same, I'd tell them he must go or you'll put him down. They usually have a lot of resources at their disposal to help them out. And he really needs retraining or he's just going to get worse. Maybe they can hook you up with a trainer, if you can afford it, or they'll take the dog off your hands. At the very least, I would confine the dog so he can't bite anyone else.
So yes, imoho, it sounds to me like the dog needs some bigtime retraining, and yes, I believe it can be done. And I don't believe that once a dog bites that he will bite again. I've seen dogs who were fear biters and will proper training they were turned around. But you have to be commited to such training, and maybe be able to afford it. And of course, you'll have to take the risk of maybe having someone seriously bit and being sued (or seriously hurting your son). But you'll already had this problem for quite some time.
Good luck and whatever you chose, I'm sure that will be the best decision for your situation.