While playing right after you meet Henry and sam I found this pic on the fridge which looks like Joel and his family and Ellie wanting to be a astronaut.
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see. Theres ellie with a astronaught and Joel with a family. You can tell its Joel because he has dark hair and is wearing a plad shirt.
Joel didn't have 2 kids.
It was just him and Sarah.
I don't think this is an "easter egg" related to Joel. It is just a sad reminder that a family that used to live in that apartment, before all hell broke loose.