Kittens with 2 Mommies???

I own 2 cats that are sisters. One of them delivered a litter of kittens a month ago today. Up until a couple of days ago, the mother cat would not let our other cat around her. But, a couple of days ago, she let her sister in with her and this cat has completely taken over the kittens! The mother now feels free to roam around the house a lot more where the "aunt" cat stays with the all the time. She doesn't leave their side and will cover them up if we come near them. We have been working to socialize these kittens for the last week or so, but new aunt cat is completely obsessed.

Is this normal? Has anybody ever experienced this before. The kittens love her like crazy. Usually, at night, there will be 3 kittens and 2 mommies. What is going on?