My little cousin acts this way? What do I do?

Situation 1:
When I got an iPhone, she wanted to see it, so I let her. She sent terrible texts to all my friends, laughing. Then, a few months later, she got an Xperia U. All my friends carry iPhones, so I've never got to check one of those out. When I asked to see it, she rudely said "no, it's mine".

Situation 2:
I have no problem with her borrowing my clothes, but she never gives them back. The clothes she does give back, however, are torn, dirty and/or stretched-out.

Situation 3:
Whenever she's upset, I comfort her. Whenever I'm upset, she laughs and walks away.

Situation 4:
She's always telling me I'm ugly, fake, look anorexic, stupid, etc. I don't usually say anything back, but once I did, she started crying and told her mother. Then, I was grounded for 2 weeks.

Situation 5:
She's always stealing money from me. She keeps whining on how she's poor and her parents won't buy her anything. I feel bad and get her stuff, occasionally, but she keeps asking for more and starts stealing stuff from my bedroom.

Situation 6:
She acts like she owns me and always bosses me around.

Situation 7:
She acts like she's better than me, flipping me off and being a diva.

Sorry if this is long. What should I do?

Added (1). I'm 15 and she's 12.

I don't know your ages. It doesn't matter.

Dump her. She is not your friend. Don't share anything. Develop new interests and new friends.

You need to lay down the law to your cousin.obviously her parents are not parenting her right and that's why she acts the way she does. You could talk to a trusted family member who will listen to you and tell them the things your cousin does.

Give her the cold shoulder. Only speak when spoken to. Lend her nothing, and if she fusses tell mom that she returns items in poor condition or not at all.
after a few weeks, she may come around and ask forgiveness and a second chance. But then again she may not. She is very self centered and does not seem to care much for you. Even though she is family, you don't owe her anything, just tolerate her but stop trying to be close.

Tyeah

So because she is family, it isn't like you can just dump her. What you can do though is cut her off. By this I mean hid your money, do not let her touch your clothes of phone, do not acknowledge her when she bosses you around or acts like she is better than you, get a lock for your door, do not buy her stuff, do not comfort her, and believe that you are not anything in situation four.
It is so hard when you have problems with your family, but if you do the following above, she will start to get the message. The longer you let her act the way she has been, the worse it will get.