An interesting side note to this thread is a couple from England who met, fell in love, and got married.
After several years of marriage, they found out that–by coincidence–that they were half siblings, as they were both conceived in vitro from the same sperm donor, as their parents had fertility problems.
Their case is being appealed to the British equivalent of the supreme court, as British law demands that their marriage be annulled . . .and that they would break the law by remaining together.