@pope_pervIncest is usually pretty clear. While some locations allow first cousins to marry—typically back when the birth and survival rates were low—the general rule is that anyone who assumes the role of parent—in any form—and has sex with the person they're parenting, is guilty, due to the power imbalance.
Now, as far as dramatic stories, some forms of incest—typically young, beautiful people who aren't yet aware of their beauty—are more intriguing than others. That's due to the inherent conflict between young, innocent and intrigued and the older, mature and trusted—who're thus seen as a 'trusted' partner, best able to guide them without taking advantage of them—beyond what the advantages they allow.
However, if that mother and son simply flirt and tease, or a man's young daughters purposely tease the father—that's generally seen as 'playful', innocent teasing—until that specific line is crossed.
Though the line between seemingly innocent teasing and taking it beyond that fine line in the sand, is particularly dramatic and intriguing, whether there's any explicit sexual contact or not.
For me, the classic Shakespearian 'boy from the wrong family' no longer means much in today's world, so nowadays, the ONLY really 'illicit' romances are the incestuous ones.