Eeeeeeeh.
Certain social setups can and do invoke a certain something in many people. Hell, the words naughty, bad, and dirty are charged under the right contexts, and incest is considered to be all three. It's also a common enough fetish to be one of the most popular PornHub queries. No one should be surprised that it's a thing.
People that have a close relationship with their family members and can't dissociate from what they're reading/watching, however, can be forgiven for being disgusted by such a thing.
I believe most people are turned on by the power imbalance/betrayal aspects. "this is a person who is supposed to protect me, instead they're eating me". More acceptable parallels lie in master/apprentice (goth milf witch anyone?), boss/employee, or teacher/student stuff.
Another angle would feed into the doting mushy kind of love boiling over into sexual territory, bearing parallels with former caretaker/ward and knight/lady (by position, not gender) or similar setups.
I've had people tell me they feel guilty, weird, or deviant for requesting such content who've expressed relief when they discover that the alternatives above gave them the same fix.
Even if fictional incest is your thing, as long as you don't act out on it, and don't bring people to harm, you shouldn't beat yourself up for it. If you're actually looking at your family members though, get some help, the issue is likely a chemical/brain one, not a moral one: you are not evil.
You know who was a real sexual deviant? The first person to have the idea of drinking cow milk. That dude must have been a pervert.
