To be fair, she dotes on Rosemary, but never harshly condemn Serena. At most, she just asks her to be nicer to her sister.
And her reaction when thinking about Serena doesn't imply she hates her own daughter.
More like she is despaired from not knowing what to do anymore. The story said she's not the type to go to social gatherings, so she probably doesn't have many friends to consult either.
Meanwhile, Serena herself prefer avoiding contact and is often helped by fact that Rosemary is taking everyone's attention.
While troubled, and annoyed by Rosemary's stupid acts, Serena still came voluntarily when her mother asks her to help with Rosemary's study for example. So I think as long as Rosemary doesn't try to pick a fight, or threatening of dragging the family into grave hole she dug, Serena can act neutrally to her. Just like they did in previous chapter.
Since it's expanding the school arc anyway, would have been better if it had taken more time to expand on the family's characters too. It feels too short, and we're back to the usual confrontation with Rosemary.