This incest case became very famous, there are reports that are still together today. They call it "genetic sexual attraction"
Genetic sexual attraction is pretty well known, although at this point it's only a theory. A very compelling one in my opinion, though.
(random example, not related to quoted post)
I also remember the case of Patrick Stübing and Susan Karolewski, siblings from Germany who met for the first time as adults, fell in love, became a couple and had four children. 3 of their children were taken away from them and Patrick was thrown in prison for a long time. The law against incest in Germany came close to being abolished as a result of the public debate following their case, but it didn't happen.
GSA occurs when two related people, who were either seperated very early on or have never met each other, meet later in life, usually as adults. GSA counteracts the Westermarck effect. The Westermarck effect is a (hypothesized) mechanism which makes us averse to committing incest. It's a psychological phenomenon, rather than a biological one. It stifles the development of a sexual attraction between people who grew up together in early childhood. So it also works on non-related people.
It is hypothesized that without the Westermarck effect, closely related people are actually
more likely to become attracted to each other, because their similarities make them more compatible. Or it's just a form of narcissism. Either way, it makes sense to me.