拝啓、妻へ。血の繋がらない妹を孕ませてしまったかもしれない…。

His parents remarried 10 years ago, and he and Hikari became siblings. Although she wasn't related by blood, she hadn't seen her as the opposite sex, probably because of her age difference. However, during the two years we haven't seen each other, her sister-in-law Hikari has matured a lot. I see Hikari as a woman now.