Jack Pearson is a hero in the eyes of his family and in the eyes of many This Is Us fans. It's hard to imagine that the man, the myth, the legend that is Jack could do any wrong. But the revelation that his troubled little brother Nicky is alive and well is about to change all of that. This twist suggests that Jack is not infallible. In fact, he may have made a choice that his children and Rebecca cannot fathom.
The midseason finale ends with Jack diving into the river after an explosion goes off on a fishing boat in the village where he's stationed in Vietnam. Nicky, who is shown strung out on drugs just a few scenes earlier, is almost certainly responsible for that explosion, and the chances of someone innocent being killed are high. We know from the future scene in the episode that Nicky isn't dead, so he definitely survives Vietnam. And at this point, the chances of Jack not being aware that his brother makes it out alive are slim. It seems far more likely that Jack disowns his little brother after the explosion and goes on to lie to his entire family — including his mother — about Nicky's fate.
In true Jack fashion, he no doubt believes this is the right thing to do. Given Nicky's state of mind, he may even believe it's the kindest thing to do. However, for his children and Rebecca, the news that their father and husband lied about something so important is going to come as a shock. Seeing them realise that Jack carried such a huge secret is sure to have interesting consequences — the most interesting of which will certainly be that the Big Three, Rebecca, and Miguel will all have to acknowledge that Jack's just as human as they are.
Because he dies at such a young age, Jack's children never get a chance to see their father as a man with flaws. Yes, he is an alcoholic, but his children all seem to understand that's a disease. Telling everyone that he knows and loves that Nicky is dead is a choice. Sadly, Jack isn't alive to defend or explain that choice. Instead, the Pearsons will have to each come to terms with their father's lie in their own way. If Nicky truly is responsible for someone's death, then their patriarch's decision to keep his brother's fate a secret will become even more questionable.
Ultimately, this revelation has the potential to be a powerful one for Jack and his children. Randall, Kevin, and Kate need to reach a point in their lives where they don't constantly feel inferior to their father. Jack needs to stop being a paragon of goodness that they feel a need to live up to. Now that they're adults, it's important for them to see their father's flaws and to face the fact that he kept secrets, did things he wasn't proud of, and made mistakes just like everyone else. These truths don't make their father any less of a hero, but they do add new shades to his character.
Nicky being alive is a game changer for many reasons. The Big Three have a living uncle they're bound to meet, and Rebecca has a brother-in-law she believed was dead and buried. But Nicky's existence is most powerful for what it represents: evidence that Jack Pearson isn't so perfect after all.