Naisho Kyouko Nee-san

Kyouko Shinonome, the strongest delinquent girl in school. She is loved by everyone as an older sister for her perfect appearance, personality, and skills. However, Kyouko has a secret as an "older sister" that she can't let anyone know about: she loves her younger brother, Ayu! (Source: Sunday Web Every, TL) Note: Chapter count includes 8 bonus chapters.