Gloria Millington was a widow who lived in the Norfolk town of Market Shipborough with her son Scott. There she worked as the office manager for lawyer Peter Kingdom.
We don't know what her maiden name is, and a visit to her father wasn't of much help as he was only referred to as Cyril.
I bring this up because without a last name, Gloria Millington could be considered the sister to Miss Kizlet, the chief mind-slave of the Great Intelligence. She worked out of the 65th floor of the relatively new London skyscraper "The Shard".
The little Miss Kizlet plaintively asked for her Mummy and Daddy, who had told her that they would be right back for her... 49 years ago.
Gloria may have been a year younger or older than her long-missing sister; there's no reason she had to be an identical twin.
The loss of the little girl had been so long ago in the past that there really was no need for it ever to come up in conversations with Gloria MIllington, not with her son Scott or her father Cyril.
'Kingdom''Doctor Who' - "The Bells Of St. John"