Kokoro wo Korosu Houhou

When his father remarries in his fifties, Haruki suddenly becomes step-brothers with Hikaru, a boy ten years his junior. Hikaru usually is a shy boy, but he likes Haruki immediately. Like an ideal little brother, Hikaru asks Haruki to play with him and spend time with him. But, Hikaru might like Haruki more than as a brother...and he might not be so innocent about it.

(Source: Fantasista Inc.)