The ending doesn't make sense? Spoilers Farseer & Tawny Man

[FoolishGirl]

Such a relationship (effectively aunt & nephew) would, while not genetically so, still by most be considered incest.
Though 'thul do not possess evidence that directly says so, all indications are that the tangle leader does considers such to be quite inappropriate. (For example to indicate such, think on her utter distaste of some sort of illegal "fan" fiction that portrays Fitz as liking children in the sexual fashion)

I don't believe that you can compare pediphilia to a non-consanguinous relationship with an "aunt" by marriage (who also happens to be about the same age as Fitz), and I don't think RH would find them similar, either. The former is revolting and wrong due to the damage it does to the child. The latter is taboo due to cultural contructs that discourage an affair of this nature due to the confusion it would cause for succession and inheritance issues.

I believe that Fitz and Kettricken, as RH has penned them, would find, due to their sense of honor, their loyalty to Verity, and their own cultural upbringing (in the context of the Six Duchies/Mountain Kingdom), that it would be impossible to act on any feelings they might have. I don't think RH's moral stance really has anything to do with it; these two characters simply would not ever "go there".

Just my two cents' worth!