Unusual spark to incest

Usually spark to incest are either discovering the intented party is sexual (little sister, mother, daughter) or coming on to in fit of lust. My suggestion is unusual. The MC is self-aware of his deeper feeling towards his mother/sister. Keeps it bottled up. Berates himself for these thoughts that only medieval hicks like the Lannisters or the paycho freaks that crop up in the news every now and then commit incest. Like his love is unique or sth.

Then he reads today's top post in Reddit. About Cheating Horror Stories in the AskReddit sub. The guy discovered his so at the time cheating on him with her brother. Dafuq?! Somebody links /r/incest below the comments. He looks at it. Shit! Everybody's doing it. Maybe even that cheater girl must be still doing it. Hell a few years back he saw this pic :

Then follows a slow and long seduction. Little by little. Like PacoFear's Stolen Kisses or GrillyTilly's It's No Big Deal.