I'm just wondering if one of them (Naruto and Sasuke) becomes Hokage, is he promoted from Genin rank directly to Hokage?
While I am not 100% sure about the transition from Genin to Kage, I do know that Gaara was at most a Chuunin before becoming the Kazekage. So, it would seem that a jump in ranks would be justified under special conditions of ability.
In any case, why should there be a need for a direct promotion? If Naruto (or Sasuke I guess) has plans to become Hokage, it does not mean that they have an intention to skip the transitions between ranks (although I'm sure there would be some sort of allowance for them to "test out").
Yes, they can.
The rank 'Genin' is usually associated with ninjas which just started out in the ninja academy or as a derogatory term for ninjas with no talent in ninjutsu.
However, Sasuke and Naruto have advanced beyond leaps and bounds. Especially now they have received the blessing we all already knew they had directly from The Sage of the Six Paths.
So yes, I think they can be upgraded from Genin (if they are still perceived as such) to Hokage.