Daniel, are you saying that when the big hen get's back from brooding (is she on a nest with eggs that your hatching or are your breaking her up?) that the little hen beats up on her for a week?
Chickens live in a hierarchal society and when one chicken leaves for a week (sometimes even just a couple day's will do it!) then the order is rearanged.. and when that chicken comes back, it is re-established once again..(maybe the little one doesn't want to give up her spot in line!?)... I think that roo's only involve other roo's and the hens amongst themselves... it's a chicken thing....
could you reintroduce her to the flock via a separate pen for a couple of days so that they can get to know her again and maybe the fighting will ease up..?
chickens are so funnnn!! Good Luck!!