Ofc, you can't seduce every single random person. Life is not a game. And asking for sex is not a part of seduction. It surely has to be a challenge if you have such a straightforward way of doing it

Dude, real-life incest between close siblings is mostly between father-daughter and brother-sister only because men tends to abuse women. Aside from that, normal relationship you could call incest is rare as fuck. You don't have be ugly as fuck to be in an incest relationship although I'm pretty damn sure it would be hard for such a person to reach 5/10 rating. But ill mentality or at least some extreme disorders are required for sure. Inbreeding in general is an extreme type of sexual disorder in most of the species. Defending it is pointless as it's not disgusting for most because it's taboo but it's taboo because it's disgusting for most. We just work like that as a specie. Your millions without some strong evidence can be considered only as a huge exaggeration. All of it doesn't work in erotic games ofc where psychological aspects can create even the weirdest and most disgusting fetishes into something acceptable (gore oO).
Part about boyfriend is not realistic for sure (tho it's pretty realistic for a kid to find his parents fucking). Still, in some crazy turn of events it's possible at least. Sexy as hell mom/sister fucking with her son/brother is not. That's just an erotic fantasy.