I have 2 children both late teenagers and trust me when I say it’s been a rollercoaster.
They start wanting to drink, to date, go to clubs, stay up late, talk back, slam doors. Having the responsibility of a teenager is pretty stressful. Plus who is going to pay for the extra mouth to feed, toiletries, etc.
They start trying to push back, they want to wear makeup, wear shorter skirts, stay in bed till midday, leave mess everywhere and never clean up etc etc. It’s all very normal stuff but it’s still tough to navigate and that’s when they’re your own, and not with someone else’s. Plus there is your other kids to consider. They will be exposed to much more with an older child around.
There is also a lot of stress with school, revision and exams and that just adds to the mix.
I’m not trying to make it sound horrendous but I am being realistic, it’s not a walk in the park. It could be a make or break situation with your niece and her parents. It’s frankly an enormous ask and I would make sure your husband considers just what’s involved. And looks like you will have the lions share of the responsibility.
Good luck in what ever you decide.