Cersei’s theory states:
“Gold shall be their crowns and gold their shrouds. And when your tears have drowned you, the valonqar shall wrap his hands about your pale white throat and choke the life from you.”
The first part has become reality— Joffrey, Myrcella and Tommen have all died, therefore they were wrapped in a “shroud,” which is cloth that a dead body is wrapped in for burial. The second half of the statement is more complicated. In Valerian, “Valonquar” means “the little brother,” so of course one would think that it will be Jaime or Tyrion Lannister who will kill Cersei (Jaime is her twin, but he was born after she was).