Am I responsible for sibling's debt?

My sister was renting our mother's house. She moved out and now I'm moving in. I went to have the electricity and water connected (they are bundled here), and they said my sister didn't pay her bills. As a sibling, I have to pay the balance or they will not start a new account for me. Is this legal? How can I be responsible for her debt? If my mother rented to a stranger, they would not be responsible.