What happens if your mother dies and she owns the mortgage on your brother's house but your brother is the only one on the deed?

Chances are that you mother owned the property, borrowed against it and then deeded it to your brother. Your brother took title subject to the mortage. The best thing to do is to encourage your brother to refinance and get a loan in his name. The existing loan might have a clause that accelerates payment in case of the death of the borrower. It is unclear if the lender would be willing ot continue accepting the payments, but your brother will lose the house in foreclosure if he doesn't make the payments.

A Different PerspectiveYou stated that your mother owned the mortgage. That sounds like your brother purchased a home and your mother loaned him the money. That means your brother owes a debt to her estate. If she had a will her property, including the mortgage, will pass according to the provisions in the will. If she died intestate, without a will, her property, including the mortgage, will pass to her heirs at law according to the state laws of intestacy. Your brother owes the balance of the mortgage to his mother's heirs and that may include himself. You can check the laws of intestacy for your state at the related question link provided below. You can add more details on the discussion page.