I'll just say that in the event of a Kyou-Kirino romantic finale, the
Westermarck issue pretty much shatters "suspension of disbelief" for me at least. They were raised together so blood, non-blood, cousins, whatever puts the notion into "0.01%" territory. It would take far more than one volume to lay the groundwork for that to be believable.
I'm not against the *idea* (e.g. I found YnS fairly believable) but I'm saying it simply has not been established in the story other than as a comedic jibe.