Bowing really isn't a good idea. It won't get the stirrup much lower and I guarantee she'd fall off the front, even if she managed to get on. Getting on a horse while they've laid down is only something I would do with a really old extremely trustworthy horse, or a very small pony. It also could create issues for the horse because trying to stand up with weight on its back is hard, and once, again, your sister will most likely fall off because most horses do not get up that gracefully. They lurch and jolt.
Get a mounting block. Or a bucket. Or a stump.
Or you could have her lower the stirrup on the mounting side of the horse to its very last hole and then readjust it once she's on.
Having a horse bow or lay down just so a rider can get on is just silly. It's not the horse's fault that your sister is short, don't make it his problem.