Ani no Shinyuu Oyome ni Oide

At age 29, Masaki isn't the least bit interested in marriage. The day that Jo, his best friend since childhood, tells him he's getting married, Masaki breaks his arm in an accident. He can't manage any of the housework, and Jo sends his sister, Ai, to come over to help. It's been 12 years since Masaki last saw Ai. He's shocked to see that she has become a tall, handsome, male high school student! Masaki is unable to hide his confusion when Ai says he has always loved him and wants a romantic relationship with him…!!

(Source: Renta!)