As
@chickens really states, it's unlikely this pullet is actually taking over the nurturing of the chicks. Here's how you can tell if this young pullet is actually nurturing these chicks or simply interacting with them on a peer basis.
A few years ago, I had a six-year old hen all of a sudden out of the blue decide to take over the rearing of four six-week old chicks I had brooded myself. She actually developed broody hormones that put her in the mode for nurturing chicks. The clue was the "broody cluck" she would emit as she interacted with the chicks. Only broodies emit this sound, which is a rapid, low key popping, sort of like an old kiddie push toy sounded like.
It's not unheard of for even young chicks to take over the care of younger chicks. Usually these are Silkies, a breed known to want to be mothers.
Your pullet isn't being "mean" when she pecks at and orders the younger chicks around. This is simply how she communicates. The chicks will let out a startled chirp when she does this, but it doesn't mean they're harmed in any way.