By the Time This Poison Spreads...

Chitose and Kairi are stepbrothers, and both have an extreme brother complex. To Chitose, Kairi is a kind, clingy younger brother, still as adorable as ever, even as a grown up. However, the feelings Kairi has for Chitose are not those one has for a brother. Kairi begins to reveal excessive possessiveness and lust, leaving Chitose bewildered.

(Source: Kaiohsha, translated)

Note: Includes one extra chapter.