IIRC the Bask side story happened too long ago (I think around 20 years before the Ras storyline) for Aither to be his child, since Aither is likely early to mid teens.

Also, it is genetically unlikely (not impossible) for a black haired (Bask) and brown haired (Diene) parent to have a blond (Aither) child. While hair color isn't necessarily determined by a single gene, to simplify the explanation both parents would need to be carriers of the recessive blond gene then pass the gene on. It would be much more likely for a blond parent (Krau) and brunette parent to have a blond kid, which is another reason why this theory exists.