The Shiunji Family Children, Vol. 1 (manga)

Arata Shiunji has never had a girlfriend, but with five incredible sisters and a younger brother, his life is far from lonely. Despite being surrounded by beautiful women every day, he begins to accept his days as a bachelor won't be ending anytime soon. But one day, all that changes when his father reveals to the family that he and his siblings aren't related by blood! Now all bets are off, and Arata's future promises to be rather nerve-wracking indeed...