you're not stupid. you didn't cause it, you can't control it and you can't cure it.
it sounds like there's a lot weighing on you and dis-ease in the home.. can you get to alanon/a 12 step group meetings? these are free and could help you with feeling guilty about your sister. im sure you love her but the best thing you can do is realise you are not responsible for her but you can be an example of taking care of you own life. i feel for you and send you lots of love energy in mkaing these courageous changes. peace be with you.