So the prosecutor alleges. The problem I have with this is that it doesn't make sense. Why would he tell him to refer to his brother as a "distant relative"? This doesn't seem like remotely sound advice for fooling a polygraph. Better advice would be "lie and pretend you don't know anyone who is into anything like that, and then use countermeasures on the control questions so that they don't know you're lying".
I mean, what scenario could they have been talking about where that advice would make the slightest bit of sense? Had he mentioned his brother before, and Dixon was advising him to backtrack ("My mistake, I thought he was my brother, but he's actually my fourth cousin twice removed")? Did CBP already independently know about him, and that they were related but somehow not know they were brothers?
I get that this was a sting, so the scenario in question was fictitious, but I cannot fathom a story the agent could have told Dixon where that would have been good advice. The only vaguely plausible explanation I can think of is that Dixon was trying to subtly trick the guy into getting caught by volunteering the connection unnecessarily.