Middle Child...My brother and sister gang up on me!?

Im the middle child. My little sister and older brother are really tight. I don't feel like I'm missing out on anything because I am pretty tight with my older brother too. On the contrary, I am really mean to my little sister. I know I should be nicer to her but she can be such a rude, egocentric brat! She's in 7th grade and I'm a senior. Sometimes I find my sweatshirts in her room! I never give her any of my clothing, and she sleeps in our big brother's tshirts and shorts sometimes since he is at college. I know that our cleaning lady doesn't put my sweatshirts in her room, meaning she must be taking them. Does this mean she is proud of me and wants to be like me? Or does she just want to show off all the cool sweatshirts from our schools sports teams that are in her home? Also, 2 weekends ago, we went to visit my brother at college. He is a sophmore in college. I slept in his dormroom and my sister and parents slept at the hotel. The whole time my brother was constantly making fun of me and insulting me. I tried to ignore it but then he brought up the fact that I was the middle child and most middle children kill themselves because they are ignored. I don't know why he brought that up but he was being a huge jerk. that night we went to a party and he got pretty buzzed. I wasn't having fun at the party since he kept telling every girl I was having a conversation with that I was a virgin and a senior in highschool...the later being true but I am not a virgin. I went back to his dorm before him and at around 3 am I get waken up by being pushed off of the top bunk by him so he can sleep there. I landed really hard on my arm but was way to tired to move even though in much pain. The next morning, I woke up before my brother and woke him up because my parents and little sister were going to be at his dorm in 2 hrs. I also wanted to tell him that he really hurt my arm when he pushed me. He first threw his pillow at me, wanting to sleep cause he was hungover, but I told him I would tell mom and dad about the party the night before if he didn't get up. He sprung out of bed and pinned me to the ground. He threatened me and said if I told our parents about last night or my arm I'd be sorry. I shoved him off of me with much struggle, really only having one arm to use and we both got ready for our sister and parents. When my sister came into the dorm my brother and her had a really long conversation about life and whatever, so I went to get some poptarts from the vending machine. When I got back my brother looked really angry and he punched me really hard in the shoulder, of the arm that I landed on when he pushed me off the bed. My sister started laughing when I said ouch really loudly. I was really angry at my brother so I inquired what the punch was about. He said it was for giving my sister a black eye. WHICH WAS A COMPLETE ACCIDENT! she shouldnt have walked infront of the soccer goal in our backyard when she saw I was practicing my penalty kick. She walked right infront of the goal when I was kicking the ball and it hit her in the head really hard, she fell down. It was all her fault because i was already kicking the ball as she walked by. Anyways I looked at my sister and said Cara, did you tell Oliver that it was an accident? and she didn't answer which caused a huge conflict between Oliver and myself, and I ended up looking bad and weak infront of Cara. I don't understand why they are both so mean to me when they are together but when they are alone they are usually nice, well at least my brother is nice to me when he is at home or if we are not in the same room together outside of home. My sister usually looks up to me but I usually just tease her and make her doubt herself. Anyways turns out my arm was broken, I went to the doctor when a day after I got home because I was in horrid pain, and I don't want to tell my brother he broke my arm because he was drunk when he broke it and it was not intentional, if he was sober he wouldn't have broken my arm. He also bruised my shoulder really badly from the punch he gave me. I'm now on good terms with my brother, last week I was still kind of angry at him but now I could care less about what happened although I still have an enormous bruise on my shoulder and a cast around my arm. I don't undrestand why my brother and sister team up on me when they are together or have seen each other recently. Do any of you other middle children feel that way? and now my sister won't speak to me because I paid for her lunch last week. I really don't understand this, why does my sister get me beaten up by my brother and act as if I'm the bad guy in the situation? And why does my brother take my sister's side??