What is not believable is that a person from nowhere might take control over the son of a powerful family, just as that son completes his acquisition of a position of power
Piers Gaveston. Elisabeth Woodville. Hans Hermann von Katte. And, at a stretch, Rasputin.
If someone speaks persuasively, flatters you and tells you what you want to hear are you really going to tell them to STFU because their family tree doesn't check out? Bollocks.