If you NR the mom cats, and release them as soon as the anesthesia wears off (ear tip them, and request absorbable sutures) chances are the kittens will do fine without her for a few hours. I would't leave the spays overnight though. I've been forced to do that a few times when cats I've trapped have milk ducts overflowing by AM! Trapped at night, spaying first thing in the AM, and let loose at no later than 2 PM, and they've been fine, and the kittens have been without them for less than 24 hours. Hope you can find out how old the kittens are that the neighbor is taking into the shelter, and hope he can clue you in as to who's kittens he took. Another alternative is to take the kittens brought in to the shelter and use them as bait for mama cat. If you put them into a small carrier, and cover it completely then place behind the trap with covering on part way over the trap back, 99% of the time the mom cat will respond to her kittens cries. Cover the trap totally after so not to alert the other cats, and never leave the trap un-attended.