We're led to believe that Tommy accepted Ellie as his niece quickly upon their return to Jackson, so I have to believe that if/when he found out about the whole 'you're not my daughter/I'm not your father' debacle, he was PISSED.
I'm thinking every time he called himself her uncle, or her his niece, or referred to her as Joel's kid, that she cringed because she was scared Joel would be upset, and one day he's finally able to weasel the story out of her. The next time she's at school or otherwise occupied, he books it to his brother and absolutely lays into a stunned Joel with something along the lines of "WHAT THE FUCK IS WRONG WITH YOU?!" and "WHY IS MY NIECE WALKING AROUND CLAIMING NOT TO BE MY NIECE??"
Bonus of Joel probably cycling through dumbfounded, shame, and regret because he was fully confident that his kid understood that he never meant it, and she is, for all purposes, just as much his youngest child as his first is his oldest. Also, Ellie is not happy to find out that her uncle ratted her out, and she now has to have a heart-to-heart between her and Joel's emotionally stunted asses while Tommy watches on from the corner to ensure they actually get their shit sorted.


















