Brain Buster

Claire was right, look in comments of the last brain teaser, and it will say the right answer. the answer of the last one is ' nothing. the yolk of an egg is yellow. '

H ere is the next one-


There was a man who was born before his father, killed his mother, and married his sister. Yet, there was nothing wrong with what he had done. Why? 


It's really hard unless you know the answer -well duh, but still -  don't spoil it!