Bran's still in full Three-Eyed Raven mode when Samwell Tarly pays him a visit in his room at Winterfell. Despite his all-knowing state, the Stark boy seems pleased to be reunited with Sam. He quickly shares the news that he's seen that Jon is actually the son of Lyanna Stark and Rhaegar Targaryen and was born in Dorne. He marvels that means his brother is a Sand not a Snow. But Bran's missing a piece of the puzzle that Sam holds.
At long last, Sam realises the significance of the passage Gilly read to him earlier in the season about a marriage between Lyanna and Rhaegar. Bran wargs back in time to confirm that the star-crossed lovers truly were married, making Jon the legitimate heir to the Iron Throne. That's not all, though. Bran also returns to the moment of Jon's birth and learns that his brother's given name is Aegon Targaryen.
Now, Bran and Sam just have to figure out how to tell Jon about his true parentage, and the fact that he, not Daenerys, is supposed to sit on the Iron Throne.