Kitten in possible trouble! PLEASE HELP!!!?

you can buy kitten bottles and special kitten milk at a pet store to feed it with. or you could remove the bully kitten for a little bit at a time (GENTLY, and with a blanket to keep it warm, and watching it carefully since its so young) while the runt kitten feeds. or you could take it to a vet and have them check for other problems besides just being small.