Lot of popular people stories find some post.
They are common enough that you know other people who have involved, whether you know it or not.
If we include first cousins, then incest is very common. It some cultures, marriages between first cousins are very common. About half of US states will marry first cousins (currently, most of those states don’t marry same-sex couples, so mostly we are talking about heterosexual relationships).
Including step-relations (stepsiblings, for example) or adopted relations, as some anti-incest laws do, then again, incest is very common.