Let your mom know that unspayed females are at high high high risk for ovarian cancer. And, repeated litters means depleated calcium= babies starve- painful way to die. and if that doesn't do it....
A few years ago, Jan spotted a cat that looked strangely lying or dragging in someones driveway as she happened to drive by a house after grocery shopping one day, and went up to the cat, found that she had given birth so many times that her entire uterus was prolapsed. In other words, it was dragging on the ground. the poor thing could hardly walk, and to top it off she was nursing! I can't remember if kittens were ever located but I know Jan brought the poor thing right to the vet and the best they could do was euthanize her. She was in much pain.