Gonna be honest here, this has confused me a bit ever since I first thought about it. But I think I may have cracked it (or, at least a little bit)

So, to start off, dainslif (or However you spell his name idk) is still relatively young, despite how long he's lived. If I recall correctly, the other sibling said that he was "cursed for failing his people"

But here's a possibility: what if that curse was not immortality, but rather to weld abyss magic for the rest of time. But as a side effect, it stops his ageing completely, making him eternally young but forever scared with the knowledge that in the end, he failed when it mattered most.

But what if the sibling had the same thing happen to her? It's possible that the reason why they're still young and healthy is because they weld abyss magic, and would also explain why we failed to go through the portal as well...

But that's just a theory

A GAME THEORY, THANKS FOR READING