Why not get together with your family and discuss what you are going to do about the financial abuse of his relatives. Not sure you can do too much about his attitude though. If you think he is fraudulently claiming benefits then report him.
How old is he? Is he on sickness benefits or on the dole? If he's on sickness benefits, I would have thought he'd have to have regular medical check ups - surely he must be diagnosed with something? If he's on the dole, wouldn't he be on Job Seeker's Allowance? Don't they try to make him apply for jobs?
Your mum shouldn't let him claim an allowance for caring - she is silly to accept that he's done that when he doesn't do anything for it. Doesn't she need the money?
It would probably cause too much of a rift if you reported him but that is what bullies rely on. Can you approach him directly and tell him you know exactly what he is up to and that you won't tolerate poor behaviour.
I wouldn't shop him yet - you couldn't prove that the 70K is his. I'd be much more devious than this (and hey its been going on his whole life so a few more months are neither here or there).
I'd be strongly advising the elderly relative to give him his money back in order to protect themselves given that he is shouting about them stealing off him. Hopefully they will be persuaded pretty easily.
Then when the money is in his own account I'd be making the call. Quick smart.
Or, alternatively (and a lot less honestly but frankly he is a cunt who deserves it) I'd leave it all as it is and then when said elderly relative dies this 70K will form part of their estate won't it and he won't be able to get his mucky hands on it!!! That might even be better revenge.
I would have to report it - you can remain anonymous, I'm sure. 70K??? How on earth has he got that if he's never worked (ggrr). Lazy arse. sounds like he's been indulged his whole life. Time for it to stop!!!