The author is preoccupied by his involuntary attraction to his married sister's breasts.  He recounts an accidental encounter where he saw her topless, noticing her breasts, milk, and overall appearance. This incident triggered an ongoing, involuntary gaze towards her breasts whenever they meet. His sister's reactions are also described: she doesn't seem offended but rather engages in flirtatious behavior, including seductive walking, touching herself, and kissing him.  He wonders if this is an instinct or a disease, and seeks interpretation of his sister's actions, particularly the seemingly encouraging nature of her responses to his gaze. He emphasizes her respectable nature and seeks a detailed explanation of her behavior, including the meaning of her kisses and seductive actions, within the context of her being a highly well-mannered married woman.  The author stresses this is a personal account and not an endorsement of immoral behavior.