The Kindle Fire is a little less of a tablet, and more a tie in for a lot of Amazon's online services. At least unless you want to root it, and then it can be turned into a respectable tablet.
The Nexus 7 would be another option you may want to consider. Full disclosure: Asus is the manufacturer of the Nexus 7, but it's cheap and is more of a generic tablet than the Fire.
So those might be where I'd recommend focusing your search. Largely because you're on a budget, and because you're sister is 8. She probably will be a bit hard on the tablet.