Hiiragi-san Chi no Kyuuketsu Jijou

Every family has its own "secrets"... Meet our protagonist, Shouta Minamoto, a junior-high student who also happens to be an orphan in a children's home. One day, he was adopted by a family living in a luxurious, Japanese-style estate. Welcomed warmly by them all, Shouta started letting his guard down, but this family had a "secret" that the world could never know. Thus begins a cozy, bloody love comedy!

(Source: Kodansha)