That and the phrasing is "the Valonqar (little brother" not "your Valonqar". So many readers speculated as to any little brother it could be ( Rickon, Sandor Clegane etc).Angelous Wang said:Cersei also hates Tyrion because of the witches prophecy that she was going to die by the hand of her little brother. Which she always assumed was Tyrion, though technically even though they are twins Cersei came out first, so Jamie is also her little brother (that part of the prophecy was not shown in the TV show though).
Then for an added twist some Valyrian words are gender neutral. Aemon said the "Prince Who Was Promised" could refer to a princess, e.g. Daenarys.
So if Valonqar is gender neutral than a "little sister" might kill Cersei. Like Arya who has her on her list.