User blog comment:B214/Question and Answers 6/@comment-2607:FB90:34E:948E:7454:613F:CFCA:B154-20200131140340/@comment-10330822-20200201012714

TBH, I think the reason is because she used to be a part of the Old Satan Faction. Remember, she is indeed Sirzechs wife, but she usually is playing the role of the Gremory Family maid. In other words, she doesn't usually play the role of Sirzechs' wife, but rather his servant.

She also distances herself from even her son, Millicas, while she is "on duty".

So I think the assumption here is that even though Sirzechs is married to her and as a child with her, and that son if obviously a full member of the Gremory family, Grayfia is probably not treated the same way because she used to be an enemy.