Responses
I think it's because Duke Rosea is known to be an ally. Even their father trusted him and his son. The Duke always smiles at the twins and he dowa not call them names unlike other nobles (at least not verbally). They don't have any idea what the Duke really is like.
Their misunderstandings with everyone is really frustrating as hell. Though I can't really blame them because of what they have experienced. I hope they will be happy in the near future >< The green hair guy deserves to be happy too, I hope the twins can somehow "save" or at least help him :<

think about it, the twins died during their high school (that's young) then they reincarnated as illegitemate children stuck in a castle (but mostly a room) for the next 5 years of their life, with nothing to learn outside that bubble. so yeah even if they are technically 20-ish they were stuck being abused for the first 5 years of their new life, so they're a bit slow on the uptake.
tbh, i dont know what im saying anymore. point age doesnt matter if your mentality never had the chance to grow, dude for all we know the could have been stuck with rurahel till they grew old, the only thing they could think about during those time were how to survive.