... in ch.4, the Bingley sisters are described as "not deficient in good humour when they were pleased, nor in the power of being agreeable when they chose it", and in ch.11: "Their powers of conversation were considerable. They could describe an entertainment with accuracy, relate an anecdote with humour, and laugh at their acquaintance with spirit."

As you say, I think Darcy would be willing to overlook quite a lot in the sister of his best friend, and I think up to now Caroline Bingley has always been at her entertaining best around him (though it's obvious Darcy knows what her real motive is). However, her jealousy of Elizabeth brings out the worst in her - her jokes take on a rather desperate edge that show her nasty streak more clearly.