Cat question after giving birth to her kittens why does my cat keep taken my other cats kittens she dont harm?

i have 2 cats they both have had kittens there is a week apart cam is my cat that give birth first and milly is the second one just befor milly gave birth she took one of the kittens from my other cats litter i removed the kitten and placed back with the mother after milly gave birth she still keeps taken cams kittens from her and placeing them with her litter cam is a good mum to her kittens yet she doesnt mind my other cat taken them why does my cat keep taken them?