As I'm sure you know, evolution works by selection.
Indeed, but there's no reason why incest would get selected out, since it's not a genetic trait.

Evolution happens by chance. A random mutation happens, which nearly always means something bad or nothing at all, but very occasionally gives the mutated creature a competitive and reproduceable advantage over others of that species. That little critter's offspring inherits that genetic advantage, and slowly dominates its species, creating a slightly improved version.

Incest isn't a genetic trait, as best I'm aware. So it's got nothing to do with evolution. You're not more likely into it if a parent was, not unless they diddled you as a kid and you liked it (nurture not nature). And you're not less likely into if if your parents are normal human beings.

Cousin-fucking is definitely more of a social taboo than an actual genetic risk. It's not illegal in the UK, let alone weird countries like Japan. Still, step-sister would definitely be lower risk (as in zero), even if cousin risk is very low.