Three words: Let-it-be
We can't change anything, if they are noe Darc's descendants, good, if they are not, good, we don't know anything yet BTW they could come with infinite explanations, and we'll hav to adapt ourselves to them, like it or not. Also, it's not that Castlevania's old story was a literature classic so chancing on thing or two is fine if it's needed for good things. Imagining our own canon is fun and useful in IGA's story because it's so empty and all of our guesses fill incredibly well, but in this one, good or not, there's no place for especulation, if they are Drac's descendants, so what? let's wait for MS to give us the reason they decided to make it like that, I know there is one.
Let it be.