Usotsuki wa Imouto no Hajimari

Mikaro is a 20-year old master thief looking to rob Tsubasa, a high school boy who had recently inherited the fortune of the Asahina group. When she finally broke into what she thought was the treasure room, all she found were girls clothes. After getting caught, Mikaro had to play along with Tsubasa's wish for a little sister and pretend to be a middle-schooler. (Source: Japanese Publisher, translated) Includes 3 bonus chapters.