How do you make my little sister feel better?

It depends if she is sad or angry. If she is sad (crying), try to do something silly to make her feel better. You can also give her something so she'll pay attention to the item, instead of her feelings. 9 year olds are interested in a lot of things. Depends on their maturity level and or their hobbies. Just give her something you'd know she'd like. If you did something to offend her, try apoligizing. You could also talk to her about any problems she wants to talk about.

Good luck!