If you have read the books, Trion was told that he was the only child of Twin and that THE MAD KING HAD AN AFFAIR WITH HIS MOTHER. This means that certain and Jaime are Targarians. John will have the greatest claim to the throne. Jaime will either die or reject it. In the books Cersai is very bitter that she lost Raygar to Lyanna. She will probably find out who John is before he does and want him for herself; to save her crown and because she wants to have Ray gars son. Littlefinger will probably tell her because he definately knows. He'll do this thinking they and Dany will kill eachother, leaving Littlefinger claim the throne. But John's lineage will be known before Cersai can marry him. Also, Cersai will get John by telling him that he'd be making peace for the realm, and that's what both his father's would have done.