Boku wa Oneechan no Omocha

Friendship... love... unrelated. Aira Aonuma is a 13-year-old who leads a school life of nothingness. One day, when their parents remarry, Misa, a high-school gal, becomes his older step-sister...! She's a completely unknown existence to him, but strangely their sense of distance grows closer...!? What's more, Misa's hobby is makeovers. She gives Aira a makeover, and he is transformed into a beautiful girl. What will become of Aira, now that he is his sister's plaything...!?

(Source: Akita Shoten, translated)