Why not try being the "man of the house"? When the welfare money comes, take it and manage it on your own. Don't let a single cent of it go towards cigarettes. When your mother has a tantrum, let her have them. Completely ignore her. She's not going to kill herself with her children in the home, no matter how much she threatens. This way, you have the extra money to go towards getting on medication, going to therapy, or whatever needs to be done to help you and your sister overcome the anxiety problems.

It really would be best to move away from that house as soon as you feel recovered enough from your anxiety to do so. If you can get on welfare on your own and move out with your sister, DO IT. Talk to your aunts and uncles, and try to make them understand what's going on. Surely they'll help you, won't they? I'm sure they'd atleast find you a place and help you move into it.