Robb's marriage to Talisa in the show, or Jeyne Westerling in the books, wasn't just a political blunder—it was the wrench thrown directly into the Stark family engine. For a family built on a code of honor instilled by Ned, it shattered the core dynamic of duty versus love. Robb was raised to be the heir, the one who puts the kingdom and his allies first. By breaking his betrothal pact with the Freys for a personal choice, he essentially rejected the very lesson his father died for. It transformed Catelyn from a strategic partner into a horrified spectator, watching her son repeat a version of Ned's fatal mistake (trusting in honor in a dishonorable game) but for completely opposite reasons. Ned chose honor over pragmatism; Robb chose heart over honor. That disconnect created this awful, silent rift where Catelyn couldn't even fully condemn him because she understood the impulse of love, but as a Tully, she knew the cost. It left Bran and Rickon's fate in the hands of a brother who was suddenly operating on a different, more isolated wavelength. The family unit, already physically scattered, lost its last shred of political cohesion because its head was no longer leading as a Stark of Winterfell, but as Robb Stark, an individual.
And you can't talk about the fallout without looking at how it redefined Sansa's and Arya's positions, indirectly. Sansa, trapped in King's Landing, became even more of a political liability because Robb's actions made the Stark cause look impulsive and unstable to the Lannisters. Arya, out in the wild, heard the news as another betrayal of the 'pack' mentality she was clinging to. The marriage didn't just kill Robb; it made the entire family more vulnerable and isolated from each other, symbolizing the moment the pack truly splintered beyond recovery. In a weird way, it's the ultimate catalyst for the younger Starks having to survive completely on their own terms, without the framework of their original family structure.