Also, I don't think mother and son relationships would be comparable to what we experience in the real world. At the age of 14, Marlene's son could have been sent to serve in defense somewhere, or perhaps Marlene's leadership role in the Fireflies kept her apart from family. You wouldn't expect that Ellie would've been having sleep overs and tea parties with Marlene's son just because Marlene was taking care of Ellie for a few years. There's a lot of possibilities for him to exist and not be mentioned previously. I think Marlene's invested interest in humanity also makes it believable and her fate makes it believable for someone to want to avenge her - Joel was an absolute monster to her. As Marlene's son, I would bash the fuck out of Joel in gameplay. My plot submission also has killing Joel halfway into the game after having played 10 hours with his killer.