she isnt mean, she is showing she is higher in the pecking order then the other.its a chicken thing that will happen whenever chickens are together.it should slack off after a little while when they both come to an understanding that one of them is the one who gets the choice food, choice scratching spot, and when the lower one learns to just walk away when the higher one comes near her.
she doesnt look thin to me. feel your birds breast bone, then feel hers and see f theres a big difference between the others and the one your worried about.
Jim