I caught my two Jersey's who are my top hens getting bullied by my geese. My guess was they were a little less fearless than my Buckeyes. So maybe like you say that's part of it since she's broody. I've only had 2 hens go broody and they were easy to break so can't say for sure.
Well she's not imprinted with your older bantam. Is the bantam getting near her eggs or sitting somewhere else? also where on the pecking order is the bantam?