Half foot cross jumps are still jumps and IMO if the horse is too much for her to canter it's definitely too much for her to jump and while it sounds 'mean' you're really just setting your sister up to get hurt.
I agree that jumping my be a little too much too soon, but you could try the around the world game. Your sister begins sitting on the horse normally, then swings the right leg over the horses neck to sit sideways. The the left leg swings around so she faces backwards on the horse. Then right leg swings again so she faces the other side, and finally, left leg over horses neck to face front. You can try this while the horse is standing or walking, but you'll probably want to hold on to him