From what I understand, this talking about how Maria and HEnry's characters are revealed when the Crawfords come to live at the Parsonage. Before, we didn't hear much about Maria. Now, we sympathize with her. She is engaged to the most boring man around, expecting to be happy, but it miserably sad instead. So when HEnry comes, she's all excited, because now she might not have to marry Mr. Rushworth. But this is where we see Henry's side. He only flirts with Maria for his own pride and amusement. He breaks her heart and later her life and reputation. This is the warning signal to Fanny. What's more amusing is that Austen made Maria's punishment having to live far away with Mrs. Norris.