Kusatte mo Ani.

Official English: College student Kirika Amano's older brother Teruo has been a shut-in for the past four years, more due to agoraphobia than anything. During that time he's been busy with his own mysterious projects, and when one day he asks Kirika for her help, she discovers that his “projects” are actually drawing hardcore BL doujinshi of his favorite shounen manga series! He's been accepted to Comiket but can't actually manage to go himself, so he begs Kirika to pretend to be his alter ego Amaterasu and sell his work. She's not sure about the whole thing, but agrees – and is thrown headfirst into the waters of fandom. It's sink or swim time…