I had a similar experience several years ago. I was feeding a stray (called Grandma now)who had arrived at my house a year earlier, pregnant. She was having her second litter and one of her daughters from her first litter was expecting too. I fixed them both boxes on my porch. The daughter had hers first. It was so funny because Grandma (her mother) crawled into the box with her and helped her deliver the kittens. Grandma had her own a few days later. They took turns nursing the bunch. The mothers moved from box to box and kept the babies seperate. After the babies were weined, I had the mommies spayed. I had never seen cats share each others babies that way. I thought maybe it was because they were mother and daughter themselves. Sadly the daughter cat was killed by a car but Grandma still visits me daily. I'd love to bring her in but 5 in the house is just too many. The babies all went to good homes.