I feed and care for about 8 stray cats. One female had a litter of 2 - three months ago. She had another?

litter 3 days ago - only one is still alive and trying to nurse but one of the 3 month old kittens keeps pulling away by the neck very aggressively and takes the baby's place nursing. I caught him several times and got the baby from his mouth as he tried to run away with the baby. I'm afraid he wants to kill the baby and is jealous. I feel bad for the mommy because I had to bring her inside with her newborn and lock her in a bathroom (with litter box, food & water) so she can care for the baby - but mommy is afraid to be inside. What should I do? Give the newborn to a no kill shelter with a nursing mother who can take the baby?