Bunkiten

Shion has an idol childhood friend and a step sister from his dad's second marriage. For some reason, he lost his memories because he fell from the stairs, so the last thing he remembered was the night of June 6th when he was supposed to buy some crepes. After getting out of the hospital, his childhood friend suddenly told him they are dating and that it's a secret from his sister. But, it turns out he's also dating his sister!

(Source: MU)