Unfortunately, there's not a lot you can do while you are living there. I would try to spend as little time in the house as possible. I had a similar situation when I was in college. In hindsight, part of me wishes I had moved out sooner. I don't think there is a way to be an adult child living at home that doesn't involve getting treated as a child!

I am not sure how you go about this, but I would really try to back away from her when she is treating you like a friend. Stop offering advice or sympathy if that's what you've been doing. Don't help her decide anything. If she starts moaning about something, just agree with her without giving her anything that feeds the conversation. When you start to get the hang of this she will (hopefully) learn that she will not get the response she wants from you.