Mary is found on the top floor of Taffington boathouse. She left with her children, Margaret and Russell, from Warwick Homestead when she became fed up with her husband's actions, after he sold their brahmin to sate his chem addiction. She and her children were killed by bloodbugs after Russell went out to try and find a chem stash he learned of from a pre-War holotape. He hoped to then sell the chems to a trader, so the family could purchase a new brahmin (the former also being killed by bloodbugs, evidenced by the notable corpse near the boat house).
pcps4 Mary's body may remain up stairs in the Taffington Boathouse indefinitely. Her body can be removed by the console command disable (opening console, then clicking her body, then typing "disable" -without quotation marks-. [verified]