What do mean by 'broom'?
Why are their back legs not being messed with? Do they always kick? That's a problem that needs to be taken care of while they are still small, or it will just get worse. Take a rope, or a long stick with a glove attached, and get them used to their back legs and under belly being touched. Your sister getting kicked in the head is not a good situation at all, it is very dangerous. If you are scared, call in a professional. Also, a rump rope will help when leading, but, you can't use that since their back feet can't be messed with. I would seriously work on those back feet before anything else. When they kick, get on to them. When they don't kick, use the approach and retreat method.
I have a 5 month old colt, you can touch him everywhere, his sheath, ears, everywhere and I don't have to worry about being kicked. Of course he was handled at birth, then his mom kept him away from us until we caught him and moved him to a small pen recently. But my mom went out there and worked on petting him everyday for about 10-15 minutes, as anything longer than that, they get bored as their attention span is short.
But you don't need to be in a rush, you can only do so much until they are old enough for saddle time.
I would not turn them loose until they are easy to catch.
I am not trying to sound mean, but those are just things I would work with first. ;)
Lettin' the cat outta the bag is a whole lot easier than puttin' it back. -- Unkown