Frail brother can't afford new primary residence but can't keep living alone. Options?

He is 58, on SSI/Medicaid, lives alone up the mountains. Wants to sell house & move into town near hospitals/family. Catch-22: Unless he immediately reinvests his house proceeds into a replacement primary residence, he'll lose his SSI/Medicaid eligibility, but the rural house proceeds won't be enough to buy a replacement house in town. Would SSI allow him to buy "in-law quarters" on a family member's property in town? Does he have any options? Thanks.