Lillian Lethbridge-Stewart

Lillian Lethbridge-Stewart (nee McDougal) was the grandmother of Alistair and James Lethbridge-Stewart, the wife of Alistair Lethbridge-Stewart, the mother of Gordon and Matthew Lethbridge-Stewart, and was the second cousin of Mario. (PROSE: The Ghosts of N-Space, Night of the Intelligence) The name 'Granny McDougal' stuck after James Lethbridge-Stewart learned about his grandmother's maiden name, and turned the song, Old McDonald Had a Farm, into a song about her. (PROSE: Night of the Intelligence)

Behind the Scenes
The story What's Past is Prologue implies that Archibald Hamish Lethbridge-Stewart is the biological father of Gordon. If true, this suggests that Archibald and Lillian may have had an affair that resulted in the conception of Gordon.