Due to no one understanding even though it comes up in most fanfiction when there is a need to explain how the two are related, I have decided to explain here quite frankly who the "cousin" is...
Even though if we wanted a proper look at things...
No I think in some long distant relationship, adoptions included (would that have been hilarious if someone had adopted a child somewhere along the way in Tsuna's side of the Vongola bloodline? XD) the two of them are more of less cousins.
Several removals and other such of course.
As well if we wanted to take into account that Xanxus is years older, and between Xanxus (at some point) calling Timoteo dad, and Tsuna calling him Grandpa (woo big spoiler for the new arc), he could be considered an Uncle.
Do I make any sense in this explanation?
Am I writing this at 1 am because I took a nap earlier and can't sleep now?