You are right, Yuva having a blood relationship does lessen the "mary sue" effect he had going for him... he was too "special" for no apparent reason, his strenght was a little out of nowhere.
But here is the thing... even with the genetics focused power system, don't you think there was a better way to convey his own potential without making him a direct descendant of the "royalty"?
What I and @
shwiftyshfifty were saying mostly is because the "descendant of a special bloodline" is a VERY overused troupe, one that as we get older we can see a... social tendency of putting people of "olden" descent into a pedestal, i wont say anymore than this because it would get overly persona, but i hope you can see where i would be going.
And as you mentioned, the powers here come from some sort of advanced genetic engineering in the far past
(and contrary to what Nihei-sensei says about the interconectivity of his work... well, we may know who was that made that engineering), that gave people of different races their traits and abilities, so think like this... what if instead of Yuva being a direct descendant of the "royal" family, he is actually a case of random atavism?
We don't really know how far and deep the genetic alterations really are in the people of this world, sure the people in power will say that they are the only ones with those powers... but you know, really, why they say that.
So Yuva is a case where the "code" that is in the genes of this world simply came to the surface with a strong manifestation of the power, same for his sister, one that... we can connect this at least... is something the "royal" family and others like them do manifest more frequently, but in reality they are not the only holders of that power.
And from here you can see the many paths.
I have my gripes with the "chosen one" and "ancient bloodlines" tropes, it is a good cliches truth be told, in a way you almost can't tell a story without a "chosen one", but many writers don't REALLY explore or diversify (with care), the possibilities that exist with these tropes.
And more than anything, these tropes are so used because of "nostalgia" in a way... we all deify the past with ease, because of "nostalgia" for "the good ol days", which then makes us believe that in the "present" of a story needs to have a direct connection with the past, but sometimes... people appear in life that have nothing connected to the past besides living lives that are consequences of that past.
I know in a narrative structure, connecting the dots is one of the basic fundamentals, but... I think there are ways of doing things that don't need that, and in my opinion, before the reveal... I was entirely satisfied with the idea that Yuva simply was a rando that happened to been born stronger. I wanted an explanation, sure, but I wanted to understand his ability in the context of this world, not that he needed to be connected to one of the already in-play factions.