My mom died and she left her house to my sister and I in her will.However, my brother lives in the house and pays the property taxes.Who is the rightful owner?

The owners are exactly who the will states it is. If the home is to be divided between you and your sister then that is who owns the home. If your brother is living there and not paying rent. Just paying the property taxes then he is gaining an advantage already. So he has no stake in the home other then what a renter would have.