Vicki, you are asking really important questions (critical thinking--yay!) about how all of this "history" is validated and authenticated. As you can see, Justin Durand is one of our wisest curators when it comes to the medieval and antiquity sources.
Every period of history has its own body of specialized knowledge and source material that scholars and expert amateur historians use to negotiate these various "truths"--but in reality, as Justin notes, it is impossible to confirm totally definite truths about what exactly and precisely happened in the distant past. Historians (professional and amateur) have to use their sharpest critical thinking skills to make judgments about which sources are more reliable, and there are often many gaps and black holes in the story--e.g. missing generations, or a lack of information about all of the children of a particular person--that leave us with gaps in knowledge.
Actually, these issues are not exclusively about the ancient past--they are still true today when dealing with families that are not well documented. And we know from even modern sources that all documented "facts" are not trues--many are assumptions, errors, or official versions that cover up unofficial folk knowledge. (I think about all the errors I have found in census records, death certificates, and other modern forms of documentation).
One of the wonderful things about Geni (and especially our curated Master Profile program) is that we try to make it clear when there is a controversy about questionable information--for example, when someone may have been the child of either person A or person B, and we have conflicting information. We need to document the conflicting information, and if both sources are equally valid, then we need to acknowledge that perhaps the case is not resolvable, but at least leave ties and links to both possibilities.