If you’re a book reader, then you know the general story of Rhaegar Targaryen and Lyanna Stark and now that the Game of Thrones Season 7 finale is finally upon us, viewers are eager to learn more about their backstory and Jon’s true parentage. But since Lyanna wasn’t Rhaegar's first wife, some fans may find themselves wondering: does Jon Snow have siblings on Game of Thrones? Now that viewers are completely sold on who his real parents are, it’s only natural to wonder if Jon has any brothers or sisters floating around who aren’t Starks — who, by the way, are officially his cousins now. So there’s that.
Before Jon was born and before all the drama between Rhaegar and Lyanna kicked off Robert's Rebellion, Rhaegar was married to Elia Martell of Dorne. She was the sister to Oberyn, who fans might remember as the poor soul whose head was crushed in by The Mountain all the way back in Season 4.
During Robert’s Rebellion, long before the events of the Game of Thrones books and the show itself, Tywin Lannister arranged for The Mountain to kill Elia and her children she’d had with Rhaegar. Her son Aegon was an infant at the time, and her daughter Rhaenys was around three or four at the time of her death. According to A Song of Ice and Fire novels, The Mountain killed both of Rhaegar's children with his brute force and then raped Elia with their blood still on his hands before killing her too. Now that whole trial by combat scene with Oberyn and The Mountain makes a little more sense to non-book readers.
Even though Jon grew up with Ned and Catelyn's children thought to be his half-brothers and sisters, Jon’s true siblings were Aegon and Rhaenys, who died before he was born. In the books, Rhaegar let his affection for Lyanna be known at the Harrenhal tournament, where he laid a blue winter rose laurel in front of Lyanna to declare her the Queen of Love and Beauty. Even though Lyanna was betrothed to Robert and set to marry him, she presumably ran off with Rhaegar, though Ned Stark and Robert both assumed it to be a kidnapping.
During the time that Lyanna and Rhaegar were in Dorne, Robert’s Rebellion started and it was Lyanna’s alleged kidnapping that was a big part of what sparked Robert’s reign of terror on House Targaryen in order to claim the crown for himself. Fans now know that Rhaegar died fighting Robert and that Lyanna died shortly after she gave birth to Jon, but of Lyanna and Rhaegar’s marriage, Jon was their only son. And as it now stands, he has no siblings currently living. Poor guy, he really can't catch a break.