@Ushkia
I do believe that that's not the case, but it could be possible, since it's said that the late king, who I'm assuming is her grandfather, married the daughter of the demon king(?), and had the ML, BUT, it could be that the ML was actually born before they married and they said it was their own, OR, the queen had an affaire and was excited because of that, and the ML was the baby of the affaire.
Though it states that the ML might not even be of royal blood, that was said by the FL's brother, so I think he really isn't and it's one of the two options that I theorize.
Ngl tho it would be w a c k if he was the FL's uncle 💀💀