Realistically he can only control his actions not the actions of others.
It sounds as though your husband has been taken well and truly for quite a long ride by his siblings.
He needs some brief legal advice firstly, this will help him feel confident about stopping paying bills for the house.
He might need clarification on council tax but I'm pretty sure it's not his responsibility to pay if he's not living there and others are. It would be a good idea to pass the details of the current occupiers to the council.
Then, help him cancel paying the bills and help him write emails and a letter to say this simply, no detailed explanation. Just the facts.
I'd also contact land registry and make sure he's registered as part owner, and put in the next level of security with land registry. This makes the legal checks harder if someone were to try to take him of the deeds as revenge....
Then do nothing.
He's in a good position whereby his actions won't affect his legal share of the house. But his actions will prompt the occupiers to pay the bills or be cut off.
The time has come to face up to his bullies. The gravy train is ending.