Bottom line, 3 options,

1) if your sister is a bitch, and treat you like crap. you should get her to pay for it.

2) If you and ur sister have a netrual relationship, pay the car urself and move on.

3) if your sister is really nice to you, and cares about you, you should tell her the truth and pay the damage to the car, and plus buy her a new cat.

I called it an accident. it wasn't on purpose, but you were a little selfish at the beginning, but i guess you probably learn ur lesson.