HIIRAGI-SAN CHI NO KYUUKETSU JIJOU

Yuuta has never had a family and lived his whole life in an orphanage, until the moment he suddenly got adopted by the Hiiragi family.
He reluctantly accepts the offer and goes to live with them without knowing what to expect from, all of the sudden, having a big family. Expectations are dashed as he finds out this family's secret...