A few days ago I was thinking what I could buy my little sister for Christmas and at first I had no idea until I called my mom and asked her for help. I still don't know what to get her but I thought I would try to give you some ideas if you are looking for a present for a girl (aged from about 6 to 12 years old). Hope it's helpful!!
You can never go wrong with a cute piece of clothing like this beanie or this cute jumper. It's quite easy to find and it doesn't tend to be very expensive.
Also, you could buy her a pair of earrings. You could get some cheap one at Claire's or if you want to spend ore you can get very expensive earring as well! There is a huge range of prices! I think these ones are adorable!!
If your sister is like mine and loves food, you can buy her some "expensive" food like some macaroons from Ladureé or if you don't want to spend a lot of money bake her something but don't buy her a banana or an orange, she won't be very happy...
And last but not least you could buy her some merchandising if there is something she specially likes, my sister loves One Direction (yeah, it's my fault) so she would be quite happy if she got that gift set!
Obviously there are many more things you could buy for your sister but here you just have some ideas!