Both her behavior and the quote prove it, to me. As you said, Mary continues to deprive Fanny of her excercise (with Edmund's assisstance!) Her statement "Knew I was keeping you waiting... Knew I was late" show a complete disregard for others' feelings or even rights. She brushes it all off with the glib "Selfishness must be forgiven because there is no hope of a cure.", as if she can charm everyone with thoughtless ditzy words. This, of course is the first time we hear this attitude, and it won't be the last.