Definitely not....not unless that mother has been in with the first mother, and even then she'll probably eat the other one's offspring. Mice are very colonial. Believe it or not, the mother will have no problem handling 15 kits, as long as she is receiving proper nutrition and can make enough milk. I have had mice in the past that have had even larger litters than that and did just fine by themselves. I would say unless you plan on euthanizing/freezing the babies you take away from her, don't bother...as you're more likely to lose them by moving them to another mother, than if you leave them with their original mom.