Family issue

Well I have a strict mom but amazing sometimes but we siblings get tired of her sometimes. Anyways, ever since my sister was on a concert she has been drinking every weekend even though she has 1 year old son. She even started to lie just to drink and I had to babysit her son, it's been almost 2 years but she still does that. Not only that but she also began to manipulate me by saying things in front of people as if I am the one lying or having an imagination even though I clearly have no interest of lying or partying. I have/had a beat friend but she also turned her back on me as she was getting closer to my sister, she was the only one who knew me well. She is also my brother's girlfriend (she is 4 years older than me and my brother's 7 years older than me). I told her everything even silly things but she has started to act weirder and weirder, I found out she is also telling my sister's everything I told her. My sister also tells her about her problems about me, then my best friend started to ignore me more and more. She even tells me to go home even though I always get permission first to be away from home. My sister is taking advantage of her but I can't do anything about it, because I don't know where to start.