Bills

So I had to move back home after I left a domestic violence situation in another state. I moved in with my mother and 5 siblings. The problem is I feel like she's asking for a ridiculous amount for bills. She wants me to pay $800 for a water bill she hasn't paid in almost 2 years and I think that's crazy. Then she cheated me out of some money she told me she was giving back but then later said I live in her house so she's not paying me anything. And now she's talking about an 900 electric bill that she wants me to pay half for. Now I don't mind helping with bills but are these amounts not crazy? It's my siblings fault because they leave the lights on literally all day and night. Plus I think it's bull she let other people live here free but try to slap me with the past due bills that they helped her run up! How can I talk to her about this in a mature way? She has a way of bringing out people's anger and it won't go anywhere if that happens. I pay for our phone bills and a little over 300 for food too.