Archibald Hamish Lethbridge-Stewart's wife was the great-aunt (PROSE: Night of the Intelligence) of Alistair Gordon Lethbridge-Stewart. According to her husband, she was a "remarkably solid" woman. The pair had at least two sons together. (TV: Twice Upon a Time, PROSE: The Note)
Behind the scenes
Mark Gatiss, who played Archibald, confirmed that his character was intended to be Brigadier Lethbridge-Stewart's grandfather. This would make Archibald's wife Alistair's paternal grandmother. However, Night of the Intelligence establishes that Archibald is Alistair's great-uncle, making his wife his paternal great-aunt. It was later confirmed that Archibald's wife was not the mother of Alistair's father, making her Alistair's step-grandmother through his biological grandfather, Archibald. (PROSE: The Note)