Why is my sister always mad at me?

Whenever I ask her something or try to tell her something that she used to be interested in she just looks at me like I'm an idiot or something and replies with her newly found sarcasm. If I annoyed her before we would just argue but now she acts like she's above everything. Sometimes I would just be sitting quietly watching tv or something and she would just grab the remote from me. If I try to take it back she would just act like she deserved the remote, she would just snap at me like an alligator or something. Before, about a year or two ago, we would have argued. Yeah, sure, I usually lost because she's bigger and older but the way she's acting now is just worse. If my friends are over playing at my house she just acts like were always doing something stupid. Shee also makes me do things for he all the time, if i don't she just starts yelling at me. She's going to be in the 9th grade next year and I'm going to my last year of grammar school.