Is my brothers wife an heir to my sisters property?

Your brother's wife is not an heir to your sister's property unless she is named in a will. Your brother might be such an heir if there is no will or if a will does not explicitly exclude him. If your brother dies after your sister died, then his estate would be an heir to her property and his widow, as heir to part of his estate, might receive an interest in your sister's property.

Consult an Attorney who specializes in wills and estates. The laws of each state can be different enough that the real answer could be different depending on which state your sister was a resident of when she died.