Clearly if you trace genealogies back far enough, almost any white male in the USA in the 1850s and 1860s would eventually find a common ancestor with any other white male. read more
Both Lincoln and Davis had their forebears come to North America from Great Britain, so somewhere they would be distantly related. They were not known to each other as relatives, only as fellow politicians and, ultimately, principal antagonists in the Civil War. read more