Your niece wants Windows. Get her Windows. No reason for anyone to force their OS preference.
Given your sister is willing to buy her a MBA. I'll use that for a price point. I think Lenovo is the best of the big brands for reliability and quality. At least for mid range prices. The Thinkpad T14 is probably the most durable. It offers excellent battery life and a decent screen. Although there are others with better screens for the price.
The difference between the two is the CPU and integrated GPU. They are upgradeable allowing more storage. You can build to order on the Lenovo website. If you wanted to get her a higher end screen. As these come with the base screen.
Buy Lenovo Laptop ThinkPad T14 Gen 3 (AMD) 6650U (2.90GHz) 16GB Memory 256 GB PCIe SSD AMD Radeon 660M 14.0" Touchscreen Windows 11 DG Windows 10 Pro 64 21CF003UUS with fast shipping and top-rated customer service. Once you know, you Newegg!
www.newegg.com
Buy Lenovo Laptop ThinkPad T14 Gen 3 AMD Ryzen 7 PRO 6850U 16GB Memory 512 GB PCIe SSD AMD Radeon 680M 14.0" Touchscreen Windows 11 Pro 64-bit 21CF003TUS with fast shipping and top-rated customer service. Once you know, you Newegg!
www.newegg.com
If you want to get her the ultimate in screens. Look at the Samsung Galaxy Book3. With a 2880x1800 AMOLED screen. It will suffer some in gaming. Due to the Intel integrated graphics instead of AMD integrated graphics. But the screen is beautiful. Battery life is good and will last the school day. But isn't as good as the Thinkpad.