GOT- The suppositions about Jon Snow’s true birth
In Game of Thrones Gilly reads the records of High Septon Maynard: “Maynard says here that he issued an annulment for a Prince Rhaegar and remarried him to someone else at the same time in a secret ceremony in Dorne. “
Last season, when Bran had the Tower of Joy vision, it basically confirmed that Jon’s parents were Rhaegar Targaryen and Lyanna Stark.
In season seven Bran’s second vision of the nuptial, where Rhaegar Targryen and Lyanna Stark (Ned Stark’s sister) legally wed confirmed the legitimacy of Jon as the full Targaryen heir.
A quick reminder: Rhaegar was the oldest son of Mad King Aerys, which makes him Daenerys’ brother. He was married to Elia Martell, though that apparently got overthrown for his new found love Lyanna Stark. Now Targaryens had a long history of incest, having spent over 300 years wedding brothers to sisters to keep their bloodline pure. This would not be unsettling for Danny, however, Jon aka Aegon Targryen had a strong moral upbringing by Ned Stark. It’s highly unlikely that he would take this shocking revelation, which makes his relationship with Daenerys incestuous. He may freak out, and I would think the latter be the case.
Either way, this piece of information certainly isn't going to make things easier for the pair. Especially since Rhaegar also was next in line for the Iron Throne… so if he had a living son… Yep: Jon/Aegon is the true heir to the Iron Throne.