User blog comment:DeathGodPrayer/Talk About What You Want!/@comment-85.154.146.101-20140209163918/@comment-1359322-20140210020012

Actually no, the Phenex Clan and Gremory Clan do in fact get along well. That's why the Gremory and Phenex Clan didn't turn into enemies.

Also you can see that the moments like Rias and Issei agreeing to help Riser with his phobia, Rias showing the proper respect Ravel's mum, who also showed that she gets along well with Rias. And the Gremory allowing Riser and Ruval to try and visit Rias during Issei's "death". So Riser's relationship with Rias' parents is purely respect to a more elderly person, despite Riser's attitude, he is still a noble, he knows the basic concept of respecting someone.