From Renta:

Mayuri's older brother Maki was a cruel and violent thug... until he got amnesia due to an accident on his high school graduation day. Now, Maki is a sweet 6-year-old girl in an 18-year-old guy's body. At first, Mayuri is pretty creeped out by the change he sees in what used to be his cool older brother, as well as his family's attitude towards Maki. But, Maki's adorable charm starts getting to him, and after protecting Maki from a bunch of gangsters who want revenge, Mayuri finds himself struggling not to feel attracted to Maki...!!
Finally, Kaname Ishida's new series featuring a brotherly love that may have gone too deep!!