Momma cat's sister helping to nurse new litter?

I have two 10-month-old cats (both female) that I adopted from a friend's cat's litter at ~4 weeks of age. Zelda and Pandora are their names, and they are both indoor cats. Both sleep in my room and so are very often in close proximity to eachother.

Pandora recently gave birth to 6 kittens of her own.

Both cats are protective of the litter, and I often find all 8 of them in a pile of licking and purring and nursing.

Even Zelda appears to be nursing them.

Is it possible that their close proximity before/during/after Pandora's pregnancy has caused some kind of hormonal change in Zelda that allows her to create milk?

Or are the kittens excersising their mouths for no reward when they suckle on Zelda (and if so, do I need to keep them separated)?

Female and even male cats can start producing milk if stimulated for a period of time. Female groups often share in the upbringing of young kittens, including nursing and teaching them to hunt.

Are you planning on spaying these babies?

Zelda...please have the cats and kittens spayed. Obviously Pandora got out and was able to get pregnant. Also...it is now suggested that cats get fixed as early as 8 weeks of age (2 monthes) since they can get pregnant before they reach 6 monthes of age. Please set up vet appointments to have Pandora and Zelda fixed as soon as the kittens are 8 weeks of age. Also...in order to keep from having these kittens have kittens of their own...please consider having them spayed/neutered as well before they go to their adoptors.

Katie