Hypothetically, if Robb had won the war and married Roslin Frey, who would have been tw best internal marriage options for Edmure and Sansa to help integrate the two portions of Robb’s kingdom?

Assuming that Robb doesn’t need external alliances, it would be a good idea to marry Edmure into the Manderlys – they’re a powerful Northern House, but they’re also the most conversant with Southern culture of any of Robb’s original bannermen. Moreover, their naval and commercial presence at White Harbor would enhance North-Riverlands connections if properly exploited. 

As for Sansa, I’m not sure who I would handle. But one idea that does come to me is for Robb to settle Harrenhal on her – Harrenhal is going to be incredibly important for him holding the Riverlands from attacks from the south, so a Stark presence there will probably be necessary for making it actually perform.