When we were much younger, we used to ask our mom why two of our cousins were so dark and she said they were just born that way.
Much later on, I found out my aunt was having an affair. My poor uncle, he treated those girls as if they were his very own up until the time of his death.
Sounds like the show "King of the Hill" where Dale Gribble doesn't realize that his "son" Joseph is the product of his wife's 14 year long affair.
As a good dad should. He was the honorable one in the family, obviously.