Behaviour Change query

Ok no advice required more just a curiosity I’m wondering if there’s a scientific behaviour explanation behind. I foster cats, currently have 2 sisters, one gave birth about a week ago. Her sister has been spayed. Sister has completely avoided kittens, would hiss and growl if near them then all of a sudden last night it’s as if something has snapped inside her, a switch has flipped and she is OBSESSED with the kittens. Every time they mew she chirrups and mews and is desperate to climb into the box with them, licking them and disturbing them and stops them from being able to eat from mum because she’s in the way which just causes more mews. I have had 2 mums at once several times before and none of them acted like this.  It’s just so bizarre how suddenly it happened and how agressive she is at trying to “help” with the kittens. It’s very cute but also disruptive.

Hello!  It sounds as if her reaction to the kitten might stem from a colony instinct, from an ovarian remnant (sometimes cats have ovarian tissue outside the ovary) or some recognition within her brain.  I hope that everything goes okay and the family grows well.