My Mother had a stroke in July 2014 and has lived with us since September 2014, after rehab. He claimed initially he was going to buy it but now he totally ignores or avoids any conversation as to what his plans are. It cost my mother$850 a month to maintain the house - out of her $1300 a month social security! The house is ALL she has.
This sounds like the brother should be paying rent that should be saved for Mom's care should her living situation ever need to change. Does he pay the upkeep, utilities, etc. Does he see Mom often? Take her to appointments? Do yard work? Does he do anything to help?
Yes see an elder law attorney to get this situation straightened out for Mom. Do not change the locks, he does have tenants rights and would have to be evicted. And one thing that is difficult for some is what would Mom do? I imagine she has some sort of cognitive issues? Or can she decide what she wants to do? Perhaps the home should be sold if mom will never be returning there.
I'd help Mom write him a letter welcoming him to the house and stating what the monthly rent will be and perhaps asking if he wants to pay it once a month or make half-payments twice a month. Establish that he has not been given permission to live there rent-free.
I'd also consult an attorney specializing in Elder Law about the best way to set this up. It may benefit both Mom and Brother to have someone living in the house, but it needs to be set up in a way that does not exploit Mom and will not be an issue in case she ever needs Medicaid.