The key thing to keep in mind is the only person in the house that isn't taking Rosemary's side is Tigur. Mom is dumb as hell, and gave up on Serena at a young age. (Like someone that got a pet, but it doesn't act the way they want - so they just get another.) The staff are either terrified of Serena, under the employ of the Prince to focus on Rosemary, or keeping their heads down and hoping to not get dragged into anything and be whipped or worse.
As others have said, once the word leaks out into gossip it spreads and mutates. Especially since mom and staff lean towards Rosemary's side, this means little to no correction or defense offered.
Lastly, when it comes to rumors and gossip, folks love nasty shit. Nobody wants to hear "the Ducal daughter was actually merciful to that maid, if you ask me", but man they love "she proceeded to whip the maid in front of her sister saying 'I wish I had taken more time killing your dog, this is rather fun' then cackling madly!"
Regarding this dragging on, this is just them building heat towards the sister so we can feel extra satisfied when she finally gets that massive cathartic payback she's got coming. Also it seems longer due to chapter splitting.