User blog comment:B214/High School DxD EX/@comment-187.195.234.94-20151015181721/@comment-187.195.234.94-20151015212720

Well friend, i don't know how answer it, it is difficult but not impossible, after all, issei is the only Guy in DxD who Sona allowed to call her by her Name "Sona", and also in DxD Ex there are the next words from one of the sons of isse:

"Zen had a wry smile and said that in reply to the invasion, the whole world cooperated to retain their peace. They fought severe battles many times...... and their father defeated them while paying Some Sacrifices."

The words "Some Sacrifices" maybe is that some friends died in battle, currently Azazel died is confirmed, Maybe Saji also died in that War, after all none of issei sons has mentioned him,  or maybe Sona only watch him as a little brother as she said and nothing more.

About Serafall, is currently confirmed that in the future she is not more Leviathan, maybe she is married with issei...