I spent several years trying to figure this out and I did not keep track of my sources because it was a personal interest, not an "argument" I was trying to make. But here is pg talking about Jessica Livingston and YC:

YC had 4 founders. Jessica and I decided one night to start it, and the next day we recruited my friends Robert Morris and Trevor Blackwell. Jessica and I ran YC day to day, and Robert and Trevor read applications and did interviews with us.

Jessica and I were already dating when we started YC. At first we tried to act "professional" about this, meaning we tried to conceal it.

http://www.paulgraham.com/jessica.html

Note: That's from November 2015. I originally joined in July 2009 and the company dates to something like 2007.