My sister and I versus our mom. Help?

You're 18. I don't see much other choice other than moving out and getting a job. By now, you should have the freedom to legally do whatever you want - I don't really understand why you're still living at home. Your mum probably isn't going to listen to much from what you've said - I think you need to bring in the big guns. Have you tried explaining this to your brothers/grandparents/aunts/uncles? And what about your dad? You might need to get an outside opinion if she is still treating you and your sister like this when you're already so old. Get your family together and confront her with this - you need to lay down the facts straight for her. Good luck with whatever you do!