That is totally irellevant for me. Not my discussion. I was just baffled that you thought that childrens are not descendants...Ahh, you edited it. So there are competing definitions, as others say grandparent or more distant.
In any case, none of those apply to Tieflings or Aasimar for a number of reasons.
"Their appearance and their nature are not their fault but the result of an ancient sin, for which they and their children and their children's children will always be held accountable."
"Aasimar bear within their souls the light of the heavens. They are descended from humans with a touch of the power of Mount Celestia, the divine realm of many lawful good deities." - A touch of power is far less than a half-celestial will have. That very strongly implies a great deal of time and generations between the initial mating of celestial and human.
Half-Celestials and Half-fiends have much more power than Aasimar and Tieflings do.
Gensasi, probably because genies are weaker than demons, devils and celestials, are the children of mortal and genie in 5e. That race makes that distinction, unlike the prior two.