Mayu-tan no Tokimeki Note #1 - Taboo ni Daite

Yayoi is a high school freshman. Her parents died when she was very little, and so now she lives with her older brothers Naoki, whom she goes to school with, and Tomoya, a medical student. One day, she learns that Naoki and she aren't really related by blood. Then Yayoi is overcome by the uncontrollable feeling that she loves him...!?