The thing about Incest:
75% of the people in the bible are related. Incest is prevalent in most relationships there, so RELIGION? Yeah, that's no excuse, and definitely not the exception.
90% of the people who act on their fantasies of being with a family member keep it secret, keep it private, and don't come out. Many move to new cities and live as lovers and 'common law' spouses for life, with no one being the wiser.
Most of those who play these games have family members they would never - not in a million, billion years to even save the human species - do these things with. IT'S FANTASY! It adds a kinky twist to the fun.
It also simplifies developer game design by having "step" family living under the same roof, so the devs don't have to create entire neighborhoods for love interests to inhabit (even though some games do that too).
With modern medicine, in 1st world nations, catching any issues with inbreeding early on is simple. In third world countries, inbreeding may be the only way to maintain the population of that society.
In the end, this is all about fantasy. Very few players will actually want to do these things with their moms, dads, sisters, brothers, nor do many players have futa love interests, want to drink or feed semen to others, want to ram their organs inside the poop-hole of their lover, want to beat them until they need medical attention just to see them get sexually aroused.
It's fantasy.
For IRL taboo, it's more common than most people wish to believe, but they would never know unless they pry into the family tree details of their neighbors.