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In her youth Cersei Lannister recieved a prophecy from Maggy the frog that she would die at the hands of the "Valonqar", which is the word for "little brother" or "little sister" in High Valyrian because of the gender-neutral nature of that language. No one person has been confirmed to fill this role; given the circumstances, it could refer to anyone who is a younger sibling.
Tyrion Lannister is the youngest brother of Cersei. Cersei treated Tyrion poorly from shortly after his birth, and suspects him of being the valonqar. Tyrion blames Cersei for Mandon Moore's attempt on his life and for Tyrion's trial.
Jaime is Cersei's twin brother, but was born after her, and is therefore her younger brother. They have shared an incestuous relationship since youth, but drifted apart after Tyrion's trial.
Aegon Targaryen, son of Rhaegar Targaryen and Elia Martell, was the younger brother of Rhaenys Targaryen. Rhaenys and Elia died at hands of Lannister forces. If the person claiming to be Aegon really is Aegon, he has plenty of reasons to want to kill Cersei.
Daenerys Targaryen was the younger sister to her brothers Rhaegar and Viserys and as one of the contenders for the Iron Throne also has plenty of reasons to kill Cersei.
The surviving Stark-children
After the death of their parents and older brother Robb all of the Stark Children (Bran, Sansa, Rickon and Arya) have reasons enough to kill Cercei. Arya has gone as far as to include Cersei on a list of people she wishes to kill.
Stannis is the younger brother of Cersei's former husband, King Robert. He is attempting to overthrow Cersei.