Miss Bingley appears 'astonished' when Darcy admits an admiraton of Elizabeth.
She makes her sarcastic jibe on a 'charming' mother-in-law.
I'm unsure if this is a sign of jealousy or of Caroline knowing Darcy so well.
I doubt she sees Lizzy as a serious rival.
JMO, Caroline has a fair idea of Darcy's general attitudes yet Darcy is a more complex personality than she realizes.