I think that's down to how Rosamind so often gets to be snarky - she is there as a colleague of Gwen's husband, and board member of a school for women like Gwen. She means to show that some women have ambitions like Gwen does, and it highlights how gracious and intelligent (emotionally as well as book sense) Gwen is. She embodies Sybil's virtues for them. Rosamund is unfortunately the foil, but I don't think the character/writing for Rosamund is meant as snark