She’s not behind academically. But how many kids do you know who are uprooted from their homes? And to go live with relatives who have toddlers, no experience with teen kids yet? Our own kids are in their same stable homes.
Gently meant, I think OP is trying to approach this based on what she thinks parenting a teen is meant to be, as if everything is normal. To do what all we parents do, to get our kids to college. It’s not normal, though. I feel OP needs to think with her heart, first, listen to the signals, make sure she’s offering the “right” support, not pressing. It’s only been since the holidays. One month or less.
The difference is if, say, her niece’s parents are temporarily working abroad or in the military. Then she’s acting per the parent wishes.