Gold digger

my sister has been married for several years now and they have a beautiful daughter, they were evicted from their home last week, he earns decent money so wat the hell? the home they were just evicted from was purchased by my dad and sold it to them very cheap..so wats the problem here? this got me asking questions around the family. it turns out her husband is a con-man or something, in summary he acts like they are poor as hell, it turns out he has borrowed £10k from my parents of which he was suppose to pay back with his inheritance that was coming...he never did he approached my auntie on the anniversary of her HUSBANDS DEATH, had her in tears and asked for £25k which for some reason she handed over. in total he has had about 50k out of my family has nothing to show for it..no car..no house...nothing. my sister has a heart of gold and seems to be completely oblivious to his obvious deceptions and is on his side, when i asked my parents what they want to do about this they told me nothing will be done because they dont want to fall out and lose contact with their granddaughter and i will also say nothing.... i am extremely angry atm and tempted to do something stupid hopefully this venting will help.... i just dont know what to do this man is going downhill and taking my whole family with him and i cant do a thing, any ideas how to handle this? thx for listening/reading