One famous case in history involved the previous titled noble dying. He'd had a son, but his son had been lost in a shipwreck, and so the title and estate went to the next in line, who was, like, a baby or a toddler or something. Except the lost son showed up one day, and then everyone spent (years? decades?) deciding if the lost son was the real thing, or an imposter who had known him in real life. The family lined up behind the Baby Heir, because the Missing Heir hadn't really been that nice of a person and they weren't interested in his return, whether genuine or not, but the public's imagination was very much fired up by the thought of the Missing Heir's Return, and it was in all the papers.
I think it ended up that he was an imposter-- he was the lost heir's best friend, and the heir actually did die, and the imposter didn't have anything to lose by stealing his identity, because he much preferred to leave behind his own real identity because Reasons.
What would happen if, in your case, the Earl was the uncle, and had a son who had died under circumstances that couldn't be confirmed, and Character A and Character B were the sons of his brother? So the title passes down to the closest male relative-- Character A-- except Character C shows up, claiming to be the lost son, except he's really Character B's puppet and has been specifically groomed for the role, and there have been a few years' worth of time (and a significant amount of money) in bringing off the charade when the time came, because they knew the real Earl wouldn't be able to have another biological son to succeed him (because of other Plot Reasons).