No one is owed a doggy bag. It's unfortunate that the OP's sister feels a need to set up a leftovers policy at all, IMO.
(For posters outside the US: I've lived in several places and never actually called it a "doggy bag." I call it 'leftovers,' and that's pretty much all.)
We went to a big meal yesterday and brought a pie and quinoa salad, but since dinner was about 4 hours later than we were expecting and we showed up an hour early due to some unpredictably smooth traffic, I ended up making a pumpkin pie while there, helping with 3 other vegetable sides, and 'making' the tofurky and veg gravy, too. Lots of work, but well worth it.
We were sent home with half a pie, half of our leftover salad (hostess took a container of her own), plus most of the leftover tofurky (2 vegetarians here vs. 1 in the hosts' home) and a dollop of mashed potatoes. Oh, and some cheese the hosts bought for before the meal and didn't like.
This is all in line with what we were expecting. If it had been my mom's house, we'd probably have been sent with some cookies and rolls and vegetables and cranberries, too, but she's very generous.