The geneticist Jim Wilson, at the University of Edinburgh, was shocked by the frequency he found in the U.K. Biobank, an anonymized research database: One in 7,000 people, according to his unpublished analysis, was born to parents who were first-degree relatives—a brother and a sister or a parent and a child. “That’s way, way more than I think many people would ever imagine,” he told me. And this number is just a floor: It reflects only the cases that resulted in pregnancy, that did not end in miscarriage or abortion, and that led to the birth of a child who grew into an adult who volunteered for a research study.
Most of the people affected may never know about their parentage, but these days, many are stumbling into the truth after AncestryDNA and 23andMe tests.
Sibling or other incestuous roleplay is super common actually. The step sibling stuff on porn sites afaik was once again enforced from their payment providers or advertisers. Because people who aren’t related, but acting like they are, are apparently turning the world into Sodom & Gomorrah, and based on some of those weird virgin or conspiracytard comments here it seems that some people actually do believe this too.