I have 5 cats 1male fixed 1female fixed 1mom and her 2daughters. The little girls are 4 months old now and he mother has stoped nursing them but they try to nurse from their fixed aunt and she lets them. She produces no milk and just gets covered in drool after20 min or so ... whats up with that?
It's like a human baby and a soother. After a while of not getting any milk they will stop.
They are trying to comfort themselves - it's OK as long as they are eating and drinking, and eventually they should stop. Our neighbor had a cat who used to suck on her arm until he bruised it, but he was way too old to need to nurse.
