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In her youth Cercei Lannister got a prophecy by Maggy the frog that she will die through the hands of the "Valonqar", what is the word for "little brother" in valyrian. The identity wasn't confirmed until now, according to the circumstance that there was no specification there could be more than just a few candidates, everyone who is a little brother to someone.
Tyrion Lannister is the youngest brohter of Cercei and her mother died while she gave birth to him. Cercei mishandled Tyrion his whole life.
Jamie is Cerceis twinbrother, but was born after her, became the second child of Tywin and a little brother to Cercei. They share a incestuous relationship since their youth, but after the trial against Tyrion they started to break.
Aegon Targaryen, son of Rhaegar, was the little brother of his sister Rhaenys who died through the hands of Lannister-forces like their mother Elia. If Aegon is really the one who he purport to be he had enough reasons to kill Cercei if he gets his hands on her.
Brandon watched incidentally a sexual encounter between Cercei and Jaime what leaded to his downfall from the tower and to his crippledness. After the death of his father, his mother and his brother through the hands of the Lannisters he would have enough reasons to kill Cercei.