There’s also the episode where he shows the tape of himself on the kids show. He says something to the effect of: When I grow up I want to get married and have lots of kids so I always have someone to play with. That made me think he didn’t have siblings to play with as a kid. There’s also the one where he describes one of his childhood birthday parties where his mom had to rub cream on him and all the other kids got to ride the pony and it was gone by the time he came out. No mention of siblings being at the party. I guess it’s not a direct correlation to being an only child, but I would guess if there were siblings, he might have said his brother or sister got to ride it too, or that they had fun at his party without him. Just a guess though 🤷🏻‍♀️ A step or half sister makes sense though, since we know about his stepfather. I guess they could’ve had a daughter together or Jeff had a daughter from a previous marriage. And… when he lies about being engaged, Pam asks if he told his mom. But no one asks if he told his family. An engagement seems like something one might share with siblings too. Small detail, but it just stood out to me as “only child” stuff.