It never said her brother would kill her actually. It said “the Valonquar” would strangle her.
“The Valonquar” just means “the little sibling”, it doesn’t say specifically it would her sibling, or whether it would be a male or female.
I had forgotten about the strangle part.
That would pretty much rule out Tyrion, or Arya wearing Jamies face.
I still believe the real Jamie will kill Cersi in the end as part of a redemption arc.
Jamie will become head of house Lanister after killing Cersi in my opinion.