You are a kid. That's ridden for how long? Last time I came on this site, boasting how good I was, after riding for 10 years, I got shot down, quickly.
Repeat after me, I barely know anything about horses. Repeat it again and again. Only riding for, what I think you said, 3 years, you haven't scratched the surface of horse knowledge. Horses are a vast topic, and you will never know everything. I almost think that you think that you know all there is to know. I used to be that way, so save yourself the disappointment I went through when I put a video up of my riding, and having people say "you're an average rider" or "go back to beginnger lessons." That was after I broke out a 3 year old, and a very green horse 3 years before.
If I was in your position, I wouldn't even think to try to teach her on a horse that also needs to learn. That is an accident waiting to happen. Get an experienced rider to get out on that horse first, and work out the kinks, and then teach the girl. (Preferably after the thrush is taken care of. Thrush buster works great). What if that horse does someting that you or her don't know how to handle? Rearing, bucking, bolting, backing up excessively, etc? She needs to ride a solidly trained horse IMO. One that she can get a good foundation on, and work on her balance/aids/etc. An unexperienced horse is not a horse to learn that on. Trust me. When you have to focus what your body is doing and what the horses body is doing, it's hard. I would rather have it where my body can hold it's own, and then deal with the horse.
My aunt is considering it, but she still thinks there is no need for a 'real' instructor -_-