...that moments before, Charlotte had dared Elizabeth to challenge Mr Darcy about his listening to her conversations. Elizabeth took the dare and confronted Mr Darcy, and received a pleasantly teasing response from him. She replied, "You are severe on us."

At that moment, Charlotte spoke up to say she was expecting Elizabeth to play for the whole party. She was standing right there, as a third person in the situation since she and Elizabeth had just been talking together. So I don't consider that her speaking up was particularly rude; she was a hostess of the party, and had a role in arranging any entertainment. I don't think Elizabeth felt it was at all rude, since her response was rather teasing to Charlotte.