Knowing you Gromit, I suggest you could do a lot of good just by being a role model. You've been there, done that, know the problems, and most importantly know that once you find your skill, and the thing you want to do most, you will find a way to do it, and do it well.
Have a chat with your niece, tell her about what you've done, what your problems were etc. Be truthful, tell her it's hard work, and there are sometimes blind alleys and dead ends, but her potential is there, and there's no reason she can't achieve it. It may be that she needs time to find her best interests and ambitions, and until then she just needs to do enough to avoid being written off too soon And indeed writing herself off. Seeing you achieve could be the confidence boost she needs.
College sounds like the best bet. If the school is failing her, it's unlikely to suddenly improve. A clean break and fresh start might be just what's needed.