She is a victim of

 

narcissistic rage. Girl A is the narcissist. Girl B and the rest are called "flying monkeys." Since they are still children, the roles may not fit exactly, but once she has the model in mind, the situation is easily explained and understood.

A narcissist preys on those who have a weak sense of the "self," because she (the narc) has an exaggerated sense of her self and sees others as an extension of herself, subject to manipulation.

Your daughter needs to build up and fortify the "self" so that her subjective-self can cover more of her objective-self which, if exposed, will be encroached up on by those who do not respect her boundaries.

Ask for her opinions on things. Give her opportunities to make decisions on her own and follow through. Respect her as an individual. Teach her about boundaries, especially the mental ones.