system
Given the following post, respond as if you're commenting on Reddit:
human
[General] Assuming the human population was reduced to 2 people, what would be the most efficient and safest method of controlled breeding to repopulate it?

Major squick warning!

This assumes that via your new culture you can convince any of the offspring to mate with anyone in the family tree. I'm guessing this would be necessary? Genetically speaking, is it "safer" for a parent to procreate with a child versus two siblings?

Or what if you have four children, two males two females, would it be advantageous to have them pair off and remain separate, or all interbreed as much as possible?
gpt
I forget what the number is, but it's somewhere around 1000.  If the number of humans is dropped to bellow that, there is pretty much no chance for survival.  
Genetically speaking, which is safer depends on how the genes spread.  You are guaranteed to have 50% in common with your parent, but could technically be anywhere between 0% and 100% with a sibling.  Genetic defects would likely destroy your attempt at repopulating pretty damn quickly.