One incident of fiction making its way into family trees has President Abraham Lincoln married to Ann Rutledge. There is little or no evidence of a relationship between these two people, much less a marriage. See Gannett, Lewis. “‘Overwhelming Evidence’ of a Lincoln-Ann Rutledge Romance?: Reexamining Rutledge Family Reminiscences.” Journal of the Abraham Lincoln Association 26, no. 1 (Winter 2005). http://hdl.handle.net/2027/spo.2629860.0026.104, (viewed 14 January 2015).
Fiction and specifically historical fiction plays a much greater roll than you might first expect in what genealogists do and how they conduct their investigations. I frequently encounter genealogists who are trying to "prove" a relationship to a famous person or group of people. This is often the case with applications to a particular affinity society such as the Daughters the American Revolution. I certainly do not include these organizations in my comments about fiction, but some researchers base their efforts to join on unsubstantiated family traditions and stories. In some cases, I have seen these efforts become an obsession.
I have mentioned one of my own family's persistent stories is that we are related to Daniel Boone's mother, Sarah Morgan (b. 1700, d. 1777). This whole story is based on the fact that one of my ancestral lines has the Morgan surname and Morgan is a very common Welsh surname at that. I spent some time researching just this point and there is no such connection but presently, the names are connected in the FamilySearch.org Family Tree yet again.
One of the most elaborate fictional efforts in my own family lines is a book entitled as follows:
Kleinman, Mary Miles. The Essence of Faith. Springville, UT: Art City Pub. Co., 1973.
All in all this is a very interesting story about my third Great-Grandfather George Jarvis (b. 1823, d. 1913) and his wife Ann Prior (b. 1829, d. 1913). It is a very entertaining book written in the style of historical fiction, which is most certainly is. However, there is a persistent family story that Ann Prior was the illegitimate daughter of an English King for which there is no support at all. The book has no footnotes and no source citations. I certainly do not fault anyone for writing or reading such a book, but there is a problem when the story starts to replace the actual history.
It is very common that a family story or tradition turns out to have a basis in an actual occurrence, but the contrary is also common. Another unsupported family tradition from my own family is the identity of a person shown in an old daguerreotype as one of my direct line ancestors. Even though my daughter and I did extensive research that contradicted the identification, a copy of the photo has now been added to that particular ancestor in multiple copies. As the animated Warner Brothers character, Foghorn Leghorn says, “Don’t bother me with facts, son. I’ve already made up my mind.”
See “Foghorn Leghorn.” Wikipedia, the Free Encyclopedia, December 22, 2015. https://en.wikipedia.org/w/index.php?title=Foghorn_Leghorn&oldid=696364795.
Many genealogists can tell stories about things they were told as children or things they were not told that turned out to be crucial issues when they got involved in genealogical research. There is no question that stories per se are valuable adjuncts to genealogical research. But as I have found personally, learning that a cherished tradition or story has no basis in fact is not a great motivation to further research and it is when the stories have replaced the history that there is a real issue.