Shounen Psycho: Ani ga Boku wo Koroshitakute Komatteimasu

Reishin became a member of the Kakuroi family, one of Japan's leading conglomerates, after his mother remarried. However, the Kakuroi family's secret business is "assassination"⁉︎ Garaku, Reishin's new step-brother, turns out to be a psychopath who enjoys killing with unrestrained abandon. Reishin and his sister Sasara immediately become Garaku's new target...

(Source: Comic Zenon, translated)