Madyline Black has always been aware of who she was - who she, her twin brother, and her cousin were. They were cousins from Sirius and Regulus Black, the brothers. Madyline was aware of the meaning of the stares she got, when she shouldn't know that. She was aware that she, her twin, and cousin were the only thing that got the Malfoy's and Weasley's agreed on -- even if no one told her, she could see it clearly.