It has been a while since I last visited this thread. Since starting this thread, I have participated in the AncestryDNA project. In my online tree, I have also continued John's line using Robert Slaugher, Jr. and Mary Smith as his parents, to see where that would take me. My DNA has since been matched with two people where Robert and Mary (Smith) Slaughter are our Common Ancestors. Additionally, I have been matched to two others using AncestryDNA where Robert Slaughter's parents, Robert and Frances Ann (Jones) Slaughter are the Common Ancestors. And, moving up the tree two more generations, AncestryDNA has matched me with another two people where Francis and Elizabeth (Underwood) Slaughter are our Common Ancestors. In all cases, for as hard as I have tried I have been unable to find any other way that I might be related to these six AncestryDNA matches.
Note that I have also noticed in 'The Parish Register of Saint Peter's New Kent County, Virginia 1680-1787' - a book which contains records relating to John and Fanny (Brothers) Slaughter - that there were Lightfoots living in St. Peter's Parish at the same time as John and Fanny. IF John was a son of Robert and Mary (Smith) Slaughter then it would have been John's brother, Francis, who married a Lightfoot.... Just a bit of food for thought.
BTW - BarbaraWalkerG - I never knew any of my grandmother's kin. She had three brothers that I know of, but wouldn't know where they or their descendants are to ask them to do the test.