My brother is the owner of the house but we pay the mortgage.
He now has put the property up for sale. What rights do I have

To move out when he sells it. Unless your name is on the deed of the house, and unless you have some written agreement as to being paid back for the money you are paying, the mortgage you are paying for him is just a gift or perhaps considered your rent if you are living there.

Without something in writing or at least some agreememt you have no rights.

You have essentially been making a series of gifts to your brother. You have no rights here unless you have some sort of side agreement with him - for example has he promised to reimburse you?