Does Atraxa count as Ixhel’s “mother”, like Ixhel counts as Vishgraz’s?I would not count Ixhel as Vishgraz's mother.
The creator/parent dichotomy is a tricky one, and for me to count someone either the creator or the created needs to view the other in that role. Neither Elspeth nor her angels looked at each other as mother/daughters. Karn, however, did view Urza as his father. It's a fine line but generally I don't view any of the Phyrexians as having that kind of role with each other. It's pretty antithetical to what they are.