I have to agree with a lot of people here, being taboo is a huge selling point, and there is also the long standing relationship you've built throughout the years that you just can't get with a stranger. but if we're talking a large age gap, like younger sister or daughter then I like the idea of like, molding them into your idea of a perfect woman, why bother finding someone when you can make your own? the love is already there just gotta twist it a bit.
Although I have to say, I'm not interested in incest outside of fiction, I just don't find any of my family members appealing.