Family issues.. Need advice on older selfish/inconsiderate/immature sibling?

Hey everyone, I'm Seth. I'll give you some background information first, I'm currently 22 and have about a year left in college so I still live at home with my mother since it is cheaper commuting to school rather than taking out hefty loans or putting it on my parents. My sister is 27 and she just got out of a 3-4 year relationship two or three months ago so she has moved back home. Now to go a little further back first, my sister has always been very inconsiderate of others in the house, she was the type of teenager that thought she was entitled to everything and would often take my (and other family member's) personal belongings without asking and I'd never see them again or they'd come back messed up, not to mention some things that were lost were irreplaceable. The worst part about it is also that if you ask her about it, she will lie and deny it to her grave and get mad that you would accuse her or she'd try to sneak it back and act like you had no idea what you were talking about (She's pretty manipulative and deceptive). Also, since she was my moms only daughter she was given a lot of leniency and got away with a lot when she was growing up so another thing she would do is bring her boyfriends home and eventually get my mom into caving into letting them stay with her. Worst thing about it is that when my mom would tell her to take them home or that there not allowed to sleep over she would just sneak them in continuously. This happened from the time she was around 17 until she was 22 with a total of at least 3 different guys pretty much moving into my family's house over that time period until she finally got a boyfriend with his own house, which was fairly annoying since every single one has been lazy. Not to mention these are some of the smaller things that she's put our family through. Now that she moved back in the house she has been going out and drinking/partying every night until like 3-4 AM and coming in drunk and waking up everyone in the house. Now here's the situation, my mom went out of the country for around week on business a couple of days ago and the day she left my sister brought her new rebound/boyfriend or whatever he is over (which I didn't have a problem with) but then the weekend was over and she still didn't take him home. So I tried saying something to her that I don't want some guy she's known for a month staying over the house like that (plus she has left him here alone while she went to work) and that I want my personal space (with no guests in the house 24/7) and reminded her that she's not the only person that lives here but she said something to the line of mind your own business but with a lot more vulgarity so I won't get into detail. So I called my mom and told her of the situation, since I knew from previous experience there's no reasoning with my sister, and she called my sister and told her that this was not acceptable at all and that he was not to stay over here. So of coarse my sister had to start a big fight over it reverting back to childhood and showing how immature she is by calling me a tattletale among other things and insulting me and telling my mom that she's moving out and is never going to see her again. So I thought it was solved yesterday and she would take him home, so I went to bed at around 2 AM after studying for my exam and to my surprise when I woke up at 8 this morning he had stayed over the night again and she left him here while she was at work again (He doesn't drive, DUI, or have a "real" job). I called my mom and told her about it and she told my sister again to take him home but I'm fairly confident that I will probably have to deal with this again before my mom returns. I personally think she did this in spite to irritate me further. So basically I need some advice on how to handle this since nothing seems to get through to her and she just throws a fit and says everyone else is wrong when she doesn't get her way. What should I do? I'm already planning on moving out next semester near my school since I can't deal with the pressure from school and her continuous drama anymore, so that is already solved. Sorry it was so long, I'm just beyond frustrated and don't know what to do.