Lizzy has not sorted all the circumstances out yet. My feeling is she essentially wants to protect Jane from unnecessary suffering. Jane never thinks ill of anyone. Lizzy is aware her sister quitely had feelings for Bingley yet Darcy's letter declares he was unaware of Jane's attachment. I wonder if Lizzy feels discomforted by her realization Ch. 36 'that Jane's feelings, though fervent, were little displayed, and there was a constant complacency in her air and manner, not often united with a great sensibility'. She may be able to see Darcy's point, though he has disrupted her sister's happiness.