Cat Desperate To "steal" Other Cat's Kittens

My mom took in 2 pregnant cats a month ago. They are a bonded pair, and have previously had simultaneous litters. This time, they are a couple weeks apart. mama #1 gave birth to 6 kittens early sunday morning. mama #2 was right there playing doula and quickly claimed a couple for herself to nurse. My mom did make sure all babies got to the correct mom so they can actually feed Over the past few days she's gotten more and more relentless, desperate to get to the kittens. she managed to bust into the closet when mom was distracted and made off with 2 kittens under the bed. So she's really had her work cut out for her, keeping the cats separate. Which is a shame, when they're bonded. Not entirely sure when mama #2 is due. her nipples look full, but her belly is small (could just be a smaller litter?) previous estimate was she was about 2 weeks behind mama #1. So the problem will be fixed once she gives birth and has her own. Any tricks, besides just keeping up with separating and waiting for her own babies to placate her?

If these catnapped kittens grow, ie fostermamma has milk, the problem isnt that big, really.

The real problem is her biological kittens wont get the colostrum which is mostly in mommas first milk.
But you can add bough colostrum.

So, admittedly, I don't know much about the cat lactation process For humans, I have gobs of experience (was breastfeeding nonstop for 10 years!) So, i would assume that the "fostermama" has a little colostrum now, but maybe not enough for her catnapped kittens to grow on? And if she did actually have enough, if cats are like humans, she won't run out of colostrum until a day or 2 after giving birth to her own, as the placentas detaching is what triggers the milk production. maybe its fine to let her nurse a couple as long as she leaves them in the main nest, so they still have access to bio mom's milk too? She may have just been sneaking off to hide with them because she kept being given the boot from the nest.

The key is to weigh the kittens daily. Easiest in grams. If they add 10+ grammes a day they get enough.

If less you must do something...

If the pair is bonded they might be used to helping each other raise kittens. I would keep the two in a confined space together so she isn't taking kittens super far and return the stolen ones to the original mama once the new kittens are born. Judy weigh everyone to make sure they are gaining weight.