Why does my cousin like this??!?

She's spoiled by her dad. He does everything for her and she doesn't even say thank you. He goes to the dinner table to get her food whenever she asks, tells him to drive somewhere to get something, treat him like a waiter and she doesn't even say thank you. Also, I'm sick of her competitive attitude. Every time we play a game and she starts losing, she doesn't even try anymore. When i start winning, she complains, starts booing, and just gives up. I always yell at her that she can't just give up every time something bad goes wrong. She's two years younger than me, (she's 10) and she always talks to my friends. I wouldn't be so mad but every time i talk to HER friends (just sayn hi, i rlly don't wanna talk to them) she questions me about what i was talking bout and says i can never talk to them. Every time i say something she thinks is offensive about her, she YELLS at me and says i humiliated her. A few years ago, (i rlly don't care about this) she had a cupcake and she let me have a bite. I didn't bite it till she was gone (she leaves it out in the open on top of a locker) and put it back where she left it. Apparently, a squirrel ate her cupcake but she didn't know so she told everyone that i did so all my friends were sayn wat a terrible cousin i am. (we all go to the same school) This year, one of her best friends called her poop but she didn't cry in front of my friends. When i said it in front of my friends, she started bursting in tears cuz i "humiliated" her in front of my "friends" (who she tries to impress all the time) They felt so bad for her, they started calln me names and one of them is a Junior Olympics basketball player so she punched me in the head. I felt like crying so bad but i didn't want to do it in front of everyone so i didn't cry till i went back home and i had a throbbing headache the day after. For everyone who doesn't no her, she's the perfect angel everybody loves but when she's w/ her family, she has temper tantrums, she doesn't answer her mom or her grandma's questions, she kicks people (me) in the stomach whenever i try to steal the remote from her and does w/e she wants. WHY does my cousin act like this??? Every one in my family says she's young, but I've been living w/ her for YEARS now and i no she's not gonna change.