My Cousin Is A Selfish Little Brat

I've been staying at my aunt's house since the beginning of summer about a week ago so I can attend college courses to get through high school quicker. She doesn't have any kids of her own so the summer time is like heaven for her I guess; she invites practically all of the kids in the family to come sleep over at her house but usually it's just me and about 2-4 other kids. The most frequent ones that come along with me are Tony and Mimi who are brother and sister. Anyway, moving on, today I spent all day driving my aunt and my cousin Tony to stores while my cousin Mimi stayed home with my aunts dog and another one of my cousins. While running around and picking some things up for my aunt in the produce aisle, Mimi kept on calling and bothering me to pick her up some Arizona but only Kiwi Strawberry or some **** and none of the ones with tea in it. I couldn't find any so I picked up some Chicken Subs from subway instead. When we got home and started eating, I asked her if she could grab me a water bottle and she started acting weird and said nothing so I decided to grab myself one from the fridge. As soon as I opened the fridge though, she came up to me and said that since she had put them in there they were all hers  there were seven bottles of water in the fridge, SEVEN. I moved to grab a soda instead but she got to it first and kept it for herself. I didn't really care if she would get mad at me anymore so I just grabbed a water bottle and went back to eating. About five minutes later she started getting moody and threatened to call her parents to come pick her up along with her brother. I honestly could not give a **** if she did call her parents and let her. As soon as her mom picked up she told them to come get them both her brother quickly responded with a no to their parents resulting in them not coming to pick them up. I walked away and went inside of my room to hopefully ignore and be ignored by everyone for the rest of the day. About thirty minutes later my aunt called me out of my room to come apologize to my cousin as she was crying from me apparently being mean to her (which was far from the truth. She was crying because I took a water bottle, because her parents decided to not come pick her up registering in her mind as a denial of a demand she made to her parents which are usually granted causing her to become a brat seeing as how no one ever says no to her.) and I honestly thought about just leaving because if she would seriously stoop to making this much drama about a water bottle then imagine what else she would do . I "apologized" and walked away to my room and locked myself away ignoring and hoping to be ignore from everyone and now I'm here seething and fuming and considering the option of quitting my summertime job, packing up my stuff, dropping out of my classes and driving back home about 3 hours up North. I honestly hate my cousin Mimi, she's a spoiled, selfish brat who is always putting up a facade of a nice 12 year old girl to everyone. But once the adults leave, she attacks her older brothers self esteem, criticizes his intelligence at almost every decision he makes and is just a plain ***** to everyone, myself included. I really don't know if I can handle 3 more months of this bullshit cause I would seriously go crazy. I go through enough bitchiness throughout the rest of the year and I really don't need that to continue into the time of the year where I can just get away from my mom and relax. There's a lot more to this story than I've put into this rant I doubt anyone would want to read it since it's way too long and I know it seems childish for me to be posting this about something that seems so trivial.