Find Ancestors

I am trying to find a Robert Tanner, middle name unknown who it is believed was an American man from Tacoma in the United States, believed to have been born around 1919. It has just been revealed to my Sister, by my Mother, who is 85, that he was her Father. The chances are that he is not alive anymore but he may have married and had family who knew of my Sister's existence. Her name is Sandra Kathleen Smith, she is now 58. My Mother's name is Mary Smith and my Father's name was Charles Smith. It is believed that Robert Tanner was stationed in London or surrounding area during the war and shortly after. My sister was born in August 1946. This is not a new revelation to my Sister as she had been told prior by relatives that her "real" Father was an American soldier in the War but this has only been confirmed recently by my Mother. She is obviously interested to now learn of any other family that she may have and any help would be greatly appreciated.