Hi Sarah. When you volunteered to take and pick her up from school, did you say "we can pick her up and take her for a week or two" etc or did you leave it open ended? She may have no idea that you are feeling inconvenienced, or unappreciated. I'm sure it wasn't her intent to pawn her other child off on you like an instant foster parent.
I can tell you as a mom of twins that had to stay in the NICU, it was quite a relief when people I knew I could trust offered to help with the SLIGHTEST thing. Even though there were only certain hours we could go to the hospital, I spent hours pumping, refrigerating, transferring milk, washing bottles, etc, and they weren't even home! My sleep schedule was way out of whack, and I didn't even have any other children at home that I needed to get ready for school, make lunches, get dressed, etc each day.
You can always back out of something... but before you create any hard feelings between you, sit down and let her know that after such-and-such date, you won't be able to, or you can only do it on Mondays, etc. It may be just that with everything going on, she trusts you and has put that completely to the back of her mind (before and after school for her other child) so the twins can be at the forefront. Maybe take a niece basket of teas, or cocoa or something over to her and approach the subject nicely. You may find out there's a perfectly good reason why she hasn't been able to take her daughter in the morning, or she may not even be aware of the fact you've helped them for a whole week already!!
Good luck, and God Bless!
Karen
P.S. mine came home mid-November, and they did just fine in the winter, as long as we preheated the car, and used several receiving blankets they were toasty warm!