Hey! Some people (not Dr. David Epstein of Columbia University fame) still think that incest is not a great idea.
There is some relief for kids who’s biological dad is a sperm donor (literally). If you know your donor number, or even if not, this registry will help you locate (and thereby avoid reproduction with) half siblings.
Apparently some sperm donors have been found to have fathered up to 150 kids, so this accidental incest is a very real problem.