Is what my landlord doing illegal?

I rent a unit in a duplex. Both units are connected by a room with a washer and dryer that is used by myself and the tenants. The landlord has all of us on one water meter. When she receives the water bill she divides it up into 1/2's and applies it to our monthly rent. There are two people in the other unit. I live with my boyfriend that is hardly here (he travels for his job ) This hardly seems fair to legal. Any thoughts??