User blog comment:B214/Volume 22 spoilers/@comment-95.90.227.74-20161018202609/@comment-9595671-20161018232344

He's basically their child indirectly, his new body was born from part of each of them.

Although ironically, I've seen signs Ophis is either a possible harem member, or she sees Issei as her daddy and both Rias and Akeno as her mommies. While seeing everyone else allied with the ORC as a sister or brother.