Joint tenancy is to do with the house deeds ie who is named ad owner and under what terms. It has nothing to do with who lives in the house or who pays the mortgage. You can check with the land registry if your mother is unsure. Do you know the history of the ownership of the house? Was it ever transferred in whole or part to your brother or does he just pay part of the mortgage?
If he is not on the deeds then your mother can leave the house in her will to anyone. If he is the sole name on the deeds it is his property. If both names are there and there is no deed of trust regarding who owns what proportion then you need to follow the advice above.