The very fact that an incest taboo exists at all indicates that it was a common thing in society. History is rife with stories of romantic relationships between family members.
I've also heard of the taboo and forbidden nature can also cause some sort of reaction for people too which doesn't necessarily mean you want it as a real desire either but can still be arousing at the time.
American society is weird in that black comedians can joke about that (and other people will laugh at the jokes), but it's still 'taboo' for everyone else to mention. There was a famous study decades ago in which researchers took a number of essays written by students and removed the authors’ real names.
I have found my brain skirting the edges of it; but I've always been able to pull back, and not fall into that headspace. I tell people even if you think a taboo thought, it's just a thought. Don't stress it! Do you think, that you have a proclivity for being the younger partner in age related sexual fantasy?