They hiss at the movement of their littermates, don't forget they can't see each other. Don't forget mum is cleaning up all their poo and wee so she will do alot of cleaning of them and some pee and poo more than others. I'd just keep an eye on the grey one if she seems to be spending more time on that one than the others.
Some kittens also have a preferred teat, so if another kitten gets on their favourite, they will push them off.