Some of my favorite lessons was when we played games. Some of these were just silly little things while the pony was standing there, but to us it was so fun. There are tons of games that help kids not only get use to being in the saddle, but also teach about balance.
One of these games we use to play was "around the world" where we would have our ponys stand, they can be held to ensure they stay put, and we would have to spin in our saddles. The first person back to the starting position wins, it actually taught you about balance and moving in the saddle. Other things you can have her do is reach for there ears while they are standing or lay back as far as they can, all of these things can help her feel more comfortable in the saddle, and helps build a bond with your pony.
It's really awesome that you are helping your little sister learn a love of horses!