Fanon talk:Mathilde Goth/@comment-11491781-20140810070058/@comment-11491781-20140811084546

And then why the heck does Katheryn take her stepmother's last name? The only way i can think of, is if Katheryn was born when Geoffrey was either romantic interest or boyfriend of Nancy, and Geoffrey + Nancy married while Katheryn was somewhere between baby and teen, but they start up married? Even if Katheryn ended up living at the Landgraab estate through story progression, it would not change the surname, right? Only if you clicked on Geoffrey and had him change Katheryn's last name.

AKA when the stepmother and father are already married by the time the stepchild is born, the surname is of the biological mother's or even the stepfather's.