Except that virtually the whole family disagrees with everything Karen does in this book.

Yes, exactly. It's unfair to see Karen's actions against the Tranters as reflecting the opinions of her family generally. The younger Marlows are surprised that their mother is prepared to go along with it. And remember the scene in the bath - it's not just Rowan and Nicola who care about Mr Tranter, they all recognise that he represents something about Trennels and it's way of life, and are moved by that.