Get multiple opinions, and look into jaw expansion
That's so sad for your niece. We could tell that my son would need braces starting when he was pretty little, so we saw an orthodontist regularly for consultations. They would have been willing to start when he was 9 or 10, but he wasn't really very motivated yet. Their faces are still growing at that age, so if she gets braces now, she might end up needing 2 rounds of them--one now, and one after her face has fully developed.
The most important thing I would say to anyone considering braces is: get multiple opinions, and tell the orthodontists you don't want to remove any teeth! We consulted with 3 separate orthodontists who had 3 different approaches to treatment. It's more like an art than a science. The last orthodontist was willing to keep all my son's teeth to begin with, although he warned us that we might end up having to remove teeth eventually. Luckily, things worked out and DS still has all his teeth.
No criticism to anyone who has truly needed this done! But orthodontists get faster results if they remove some teeth at the beginning, so I think they're prone to recommending this even if it could be avoided.
If I were you, I would also try to find someone who could work with your niece's family to help her jaw grow and expand more. That would give her more room for her teeth. I read about Lightwire appliances, and I still don't know if that's a scam, but if you google it you can learn more about jaw expansion. Anything they can do to help her jaw expand even a little more than it would otherwise will help her teeth in the long run.