I believe you can sue you bro,
bcauz there are laws which protect people from cheaters,
if there are no lawyer on behalf of your parents,
you still can file it as your parents is being cheated to sign these documents,
without their conscience.

Because I'm also living with my bro,
and I also inherit a fortune from my parents,
and I heard this from the lawyer tell me no to sign any suspicious document,
after signing it still can file and get it back but hard.
But i believe my bro won't do this to me.