This is a hard one but I think I might get what you're saying. I am an older sister with a younger sister who I feel takes advantage of our mother. I am now 38 and she is 36. I have always been independent and even when times were tough, I made it through. However, my little sister is still living at home, has our mother raising her daughter and has taken over the house. Needless to say, it makes me furious to watch, hear and be around.
My sister is a manipulative, mean person...to everyone. I remember trying to get my mother to see it and put her foot down, it got me now where and only caused drama between us. My mother started to push ME away, not my sister.
So here is what I've learned for my personal situation. Get over it. Let it go. Yes, it sucks, but it is your mothers choice and she is her daughter. You need to deal with YOU. Not your sister, not your mother, not the entire family. Just YOU. Take care of yourself, find a way to cope with the situation (meditation, yoga, reading, get a hobby) but LET IT GO. It is not your job or responsibility to "mother your sister" by trying to control the situation. You have a choice when there is a fight or drama. You can get pulled into it, create it or walk away. It's not worth the walls it will build. She's 10 years old...what she is doing is mostly normal. Annoying and upsetting, but normal for her age.
And think about it this way, how do you think it makes your mother feel when your sister is upset and you are upset...at her. I'll tell you how she feels, like crap. As the eldest child she needs you to be supportive, kind, understanding and maybe take her our for lunch or a movie now and then. Not a lecture on parenting...you're not a parent. I have this awful image of an 18 year old young WOMEN whining to her mommy about her 10 year old baby sister...not cool. Not cool at all. Why are you letting a 10 year old upset you that much? You're legally a grown women now. Let it go and be thankful that you can get up and leave when ever you like. Your sister is still 8 years away from that. You're the lucky one :)
Best of luck either way and remember, your freakin 18...go HAVE FUN!!!!