Mr Bennet and his daughters (particularly Elizabeth and Jane) felt ashamed of Lydia and Wickham - why?

The marriage of these two underscores Austin's themes of superficiality and social entitlement. Lydia gets a husband who is deceitful in both matters of the heart and business. He has little regard or conscience for others. Lydia is dazzled by the promise of money and her mother's stamp of approval. Her illusions of social respectability and wealth take the place of any sort of real love. I think the difference lies in that while Wickham knows he is a scoundrel, Lydia is rather oblivious to everything.