Topic comes up here every now and again. I have never had an incestuous thought about a member of my own family for real, but I pretty much only play games that feature (or at least have) incest themes. I forget who the poster was who said it, but I play these games so I can do all the terrible things I'd never do for real. And that goes for the real porn stuff, too.
From a hetero-cis POV, I think there's an innate attraction of males to mother figures and girls to father figures, and I've said before, but I don't have have siblings, yet can 100% find it believable that two opposite sex kids under the same roof, 14 or 15 months apart in age and hitting puberty at exactly the same time might reasonably get curious about each other. (In that case it's less taboo than it is convenience, at least to my reckoning.) A big part of the taboo comes from the age/authority imbalance that exists with a parent/child relationship, but that also exists with teacher/student, Dr/patient, etc.
Growing up I had friends whose moms were single (and open for business *nudge*wink*) and a couple of them were smoke bombs... one in particular was very well liked by the guys +/- a year or so of my age. And her daughter (who was super cute in her own right) was my first roommate as a college student. We had been platonic friends for nearly our entire lives, but after a couple weekend visits by her mom where the three of us would go out for lunch, spend time walking on campus, whatever..... man, did my mind kick into a sullied and impure gear. I was 19, she was 18, her mom was in her early 40s I think, I thought we would've made a wonderful smutty little tercet if it came to it. That year was probably the start of it for me from a fantasizing perspective.
I'm now old enough that the attractive girls I grew up with are grown women with college age daughters of their own. And a few of those girls are epic. (One's a gymnast/dancer, another's a semi-pro waterskier, so they are properly fit little bundles) And the thoughts that have filled my head are just.... sometimes I am amazed at my ability to amaze myself.