Headcanon: Mrs. Turner’s married ones are Sherlock and John from the future. The atomic bombing on WWIII made London inhabitable. Mycroft, of course, was able to get our gay babies to safety before the bomb hit. When the men were left sadly pining for their lost home and landlady, Mycroft let them in on the extremely classified information of time travel so they could return to London. So as to not be too suspicious, when they return to their past, they move in next door to their beloved flat.