Actually we are not if Yona is a direct descendant since so far it only mentioned that Yong-Hi is the direct descendant. Either Yona's mum is from the same blood line (Since she reminds me of Hiryuu's son) or Yona was born without any blood of Hiryuu. The latter would be an interesting take on how the power struggle between the descendant and reincarnation would play out.
Well, reincarnation doesn't have to be of bloodline, since it's the soul that come back. It would be a different thing if it's a reincarnation through resurgence then Su Won would be it.
Besides if Yona was part of the bloodline then she would have the illness and die early. So I get the idea of her not being a direct descendant by blood.
So I was partially correct with my earlier assumption. Yona IS the reincarnation she has Hiryu' soul and SooWon has his body/blood since he is a direct descendant of Hiryu. Yona is a direct descendant of Hiryu but the blood manifestation is stronger in SooWon.