An I mentioned that for this to work, Kelmomas' wife must be Anasurimbor as well. Which implies incest in the family.
Not really, all you need is for NC to be officially acknowledged to be considered an Anasurimbor, you don't have to have his mother be an Anasurimbor as well. So long as Celmomas is ignorant of the affair, Nau Cayuti is an official Anasurimbor and secretly a bastard. Celmomas was cuckolded, but presumably did not know it.
But that point is moot because Nau Cayuti is not Ganrelka's heir.
We know from the books that Celmomas' ancestor divided his kingdom amongst his three sons, Aorsi, Kuniuri, and Sheneor. All three sons ruled these respective kingdoms and all three sons kept the name Anasurimbor.
The Darkness that Comes Before tells us in the prologue that Ganrelka, the last king of kuniuri dies, survived only by his bastard son. Presumably, this is where the name Anasurimbor entered the Dunyain+Bastard population of Ishual, and the name Anasurimbor survived there because the bastard called himself an Anasurimbor (unless another Anasurimbor came to Ishual with the Dunyain or after the Dunyain arrived at Ishual.)
The appendix to TTT tells us that Ganrelka was born when Celmomas was fifteen. This means he was conceived when Celmomas was fourteen.
This means that Ganrelka is either Celmomas' younger brother or Celmomas' eldest son (as it seems extremely unlikely that another son would be older).
The appendix also tells us that Nau Cayuti was Celmomas' youngest son, which means Nau Cayuti has at least one older brother.
It is also possible that Ganrelka is not of Celmomas' line at all, but of the lines of one of the Anasurmibor kings of Aorsi or Sheneor. However since he is discribed as being of Kuniuri this is extremely unlikely, unless he knew all the other Anasurimbor kings were dead and believed himself to have inherited the king titles from aorsi and sheneor. Again, that seems unlikely.