I think I'm not going to get my sister what she wants for Christmas.
Why would you not get your sister what she wants for Christmas? It seems selfish.
It's a long story, but the short reason is because she is a bitch.
I see you dislike your sister but it is expected of you to get her what she wants for Christmas. If you don't get her what she wants for Christmas she'll be very upset.
I have a good reason for being a jerk to her. She's a second rate person and I always have to pick her up from school.
It sounds like you are frustrated about having to be responsible for her. I guess I can see where you are coming from, but why punish her for something she has no control over? I'm sure when she gets older she will have plenty of chances to help you out, so I think you should get her the Christmas present she wants because that shows how awesome of a sibling you are.