Hmmm...I'm trying to think how I would write it if I was going to and it's actually pretty hard. Harder to keep it IC for Frank imo anyway.

Heh, haven't had much experience with incest in any of my fandoms *ponders* but I guess it's hard for it to come across as realistic with the majority of characters because how many characters would realistically do that? It's a pretty big taboo. I'm of the mind that if you write it well then almost anything can be done convincingly but it'd be a task and a half in a lot of cases!