According to elven myth he is the son of Mythal and Elgar'nan. Distinct because they are not really brothers but have a bond that has no parallel in Thedas now.
*blinks*
Ok, but- as posted earlier, Dalish myth and reality have a really loose relationship, when they coincide at all. Remember, we're not talking about actual Gods here- we're talking about a ruling class of mage-kings. They had inter-alliances, yes, but I think you're taking the "family" relationship as presented in Dalish myth a wee bit too literally.
As for the "bond that has no parallel in Thedas now": that's a reference to the fact that Falon'Din is Dirthamen's fade-self, which is as much a part of him as his shadow. Dirthamen is Falon'Din's source, if that makes any sense. My own theory is that when the veil went up, for some reason all other mortals were partially severed from this "shadow" except for him, making him unique in the modern world, but representative of what might have come before.
This makes me suddenly feel really, really weird about the pieces of fade that come through for mages, calling their name and begging for attention. Scraps of their lost... fade shadows? Trying to reach through? Oh no. No no no. Stop the theory before it starts. Walk away.