Wow, Flemming. :D
Suzanne, since Martin Pedersen seems to have no brothers or brothers-in-law who are candidates to be uncle Viggo, then if there is an uncle Viggo, he has to be a brother (or brother-in-law) to
Karen Marie Henriksen born in 01 August 1870 in Hvorslev, Houlbjerg, Viborg (ops 49).
However, a person might easily call his brother "Uncle Viggo" when speaking to his children and others, so perhaps you could investigate the family story a bit to see if Viggo could be your grandfather's brother rather than his uncle.