If you do anything before consulting with an attorney first, you are risking making a huge mess and causing problems with your sister's care. It get it, you don't like the dirty tricks her husband pulled and you just want him in trouble. However, if you go about this the wrong way, its your sister that will get hurt.
I would think first talk to a lawyer--if she's doing better in a nursing home, then I wouldn't want to screw that up for her. See what they think the consequences might be for your sister if he is found to have done something that might disqualify her. There might be something that will help an innocent spouse, professional advice is the way to go.