[kuroishinigami]

Yea, i'm talking about vol 4.

Spoiler for :

akito found out about his real parents from nikaidou-kaichou. IIRC, his actual parents are relativ of nikaidou-kaichou.

Sorry about the misleading part. The new harem member is the daughter ofthe family that adopts him when he's separated from akiko. I don't kow how you call such sister beside step-sister. Adopted sister maybe?(which also not exactly true since akito is the one being adopted instead of said sister).