I'm not totally clear whether your DN is your DSIS only child or has siblings but you say in your first post they already have a laptop they use for school, so it sounds like it's shared whether between DN and siblings or with her parents. So it's up to them when they decide to replace it and what with.
She has use of a laptop for her education so isn't going without the tech she needs. It may be running slowish but then my DH manages to work from home as a website and graphic designer on an old laptop work haven't upgraded yet. Any working laptop will do for secondary school work.
My kids will always share a device. Electric bill is high enough with DH working from home, then I, he and eldest have our own (cheap, android) smartphones Younger 3 share a tablet, will upgrade that to a laptop when at secondary. Then another three smartphones to charge when younger DCs that age is going to be bad enough on electric bill plus limited charging points in our little house! Software, printing can all cost. They will have to continue to share, anything else isn't practical. Don't know where we'd put six laptops/tablets!
My extended family wouldn't understand this fully with!larger homes and electric bills they can afford which are higher than our mortgage and utilities combined.
It's so lovely the amount of time your DN has spent with you over the years and that you've been able to provide that support to your DSIS when she's been working. But this is a call for her to make as Mother, not you as Auntie. You do need to just quietly return the laptop, can you really not afford to put £20 in a card for your niece before returning it... I say that because if you have the budget to afford presents like that for DCs that aren't your own as well as yours then I'm expecting you can.
I hope your baby is home for Christmas and your other DC stays well. Must be a very difficult time and do feel for you but sense you do need to drop this straightaway or it could cause problems with your DSIS and relationship with DN going forward.