Ani-Imo, Vol. 2 USED MANGA

If I cheer up, will you do what I say for five minutes?

When elder brother Youta and kid sister Hikaru get into an accident that switches their bodies, Hikaru (in her brother's body) changes into a fiendish letch, and Youta (in his sister's body) finds himself her "girlfriend." Thrust into a bizarre reality where he's now taking orders from his own body, will Youta ever get himself out of this pinch and back where he belongs?!