My sister's cat keeps kidnapping the other cats kittens

(the mom to the kittens is now fixed she wasn't supposed to be outside with a male but slipped out during a school bus run. They didn't think she'd been out long enough but she still got pregnant but didn't show much and surprised them with several)

That cat isn't fixed but they were told by her previous owner she can't go under anesthesia. There's no males old enough to get her pregnant and she's not pregnant (they checked with the vet). But she keeps grabbing the kittens and dragging them inside a box and not letting them out. It's becoming a problem but there's, as far as anyone is aware, no reason for the behavior.

Is there a way to get her to stop? I tried helping but I can't figure out why she's doing it. They are worried she's gonna hurt the kittens with how she's dragging them. She keeps grabbing them by the back or face not the scruff.