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as far as the older kitten stealing colostrum, you are uneducated about lactation in both humans and animals otherwise you wouldnt comment as you have been. the mother will continue to make mature milk for the older kittens and then colostrum will start to come in when the next litter is ready to be born. colostrum even if nursed by the older kittens will continue to be made by the mother until the next litter is ready for the more mature milk. there is no limit on how much colostrum or milk any animal or human can make, it is quite simply a method of supply and demand. the more milk removed from the breast or teat the more milk will be made at future feedings. good nutrition and good hydration go a very long way in maintaining a good supply.
These aren't even for certain her kittens (and again I doubt it highly). If you read, they ASSUMED they were possibly from a past litter merely from the fact they are suckling off her. Which even the previous owners seem unsure of based on what has been said. I've been around cats and kittens my entire life and have seen full grown adult cats(at least a year in age) attempt to suckle off pregnant mothers - successfully! Of course, they were separated as these should be if the kittens have already been weaned(and also, assuming here but since they were taken in as separate "additional cats" chances are they were).
Honestly I dont want to come across as confrontational, but if the mother doesnt want to nurse the older kittens then she sure as heck will not let them. its not strange at all for her to nurse them even if they are not hers, mammals do not only nurse for nutrition but also for comfort, bonding and familiarity.
My own cat that my in-laws so nicely put outside just days before she was due to be fixed had a litter in our house. Each baby weaned at a different time and when they did so went to a new home. The last baby was far bigger then his mother and both were neutered. Still she bathed him and he would nurse for a few seconds before going to bed at night.
With our horses we have found that the babies force weaned between 4-6 months are much harder to train, had more nervous issues and bad habits and took longer to get things then babies weaned by their mothers or at much closer to one year. If the older colt was still nursing or with his mamma when the next was due we would separate temporarily only for the fact that the one year old would often play too rough with the new baby. After the new baby grew up if pastured together sometimes mamma would let the older colt nurse a little or not at all. It simply depended on the mare. Also has to be said that the mares lacked nothing nutrition wise, quality hay available free choice, good pasture, specific pregnant and lactating grain with vitamin supplements.
Op, I did read the post with you saying you plan on taking care of them. The cheapest option to care for would be to get already fixed cats and rehome them. Then you can maintain the exact numbers you want with out the hassle and expense of fixing kittens. Your mammas could have two babies a litter or twelve, theres no way to determine.