@deerafl
@Neltharn
@Silwith
TL;DR at bottom
Okay, so chapter 2 only has Margrave Gordo as a "relative" which can mean anything. So the only thing to go off of is Gordo's rank of Margrave.
In short, he's a military governor tasked with guarding the borders of the nation; hierarchy wise he's higher than a count. We also saw that this world is in conflict, cause the Empire to the north is having designs on Gordo's territory, so Gordo is likely highly trusted. Historically, Margraves in conflict zones had massive amounts of political and military power and autonomy available to them, and essentially could act as another head of state (they could conquer land and keep it for themselves and distribute it to vassals, so long as they were loyal to the crown/emperor).
That being said, they do not outrank a Duke. And duchies tend to be reserved for direct blood relations to the crown (brothers and sisters). Marquis is a related rank but depends on the system; in the Holy Roman Empire, Margraves had the authority of the Emperor, but Marquis were simply noblemen who ruled the borderlands and was used more in Italy or Spain.
To compare, a duke ruled a duchy that was a province, and counts ruled counties within a province, with barons ruling smaller lands or cities within those counties. A margrave's territory could rival a dukes, depending on the circumstances, but would never be smaller than a count's.
Basically, Margrave Gordo
should be at the closest a 1st cousin and no farther than a 2nd cousin. There could be a large age gap between the king and Gordo (hard to tell, being a ruler has lots of stress with it) so Gordo could even be once removed.
TL;DR
Gordo is either a First Cousin of the king, or at the furthest a Second Cousin, Once Removed. Highly unlikely (but still possible) to be a brother to the king (relative is a cold way to refer to a sibling).
Which makes our MC and Char far enough apart that it doesn't legally count as incest in our modern times. The incest would only be in their minds as Char thinks of him as a real onii-chan and our MC see's her as a true imouto.
PS: compared to modern times, the mayor of Los Angeles would technically be a Baron, but given the strategic importance of the city and it's port, the mayor would have the equivalent power to a Count or Viscount.
San Diego and it's county however, would have their mayor qualify as a Margrave, since it's right at the border with Mexico.