Isn't there something in the witch's prophecy that may describe just that?
In the books at least, the 'prophecy' tells her to beware the
valonquar, which she finds out his High Valyrian for 'little brother'. So it's one of the things that drives her paranoia and hatred / fear of Tyrion, and ultimately helps her on the way of going mad. She thinks Tyrion is going to kill her.
However there are many theories about it. The four most interesting being:
1. It refers to Jaime. They are twins, but she came out first, which makes him a little brother too.
2. It could refer to ANY OTHER CHARACTER who is a younger brother of someone else. Possibilities could be Tommen, Loras Tyrell (who has two older brothers in the books), Stannis Baratheon (who is still alive in the books), Bran Stark, Euron Greyjoy... The list goes on.
3. 'Little brother' is just one way of translating the word. It could mean 'little sibling', as apparently High Valyrian is gender neutral. (I think?)
4. Prophecy is bullshit, and it means nothing except to make her paranoid!!!!!
I can't quite recall exactly what the witch woman said to her in the show.
Anyway, Jaime killing her would be a pretty fitting end. I was kind of disappointed that her wildfire explosion just blew up the Sept, I thought the idea was it would spread.... like wildfire? And consume all of King's Landing. And Jaime would have to kill her to stop it happening. Still could possibly happen later on though.
I sure hope their relationship becomes much more strained and difficult now. One of the most important things in Jaime's change of character in the books it that he has effectively broken up with Cersei, in a really bad way. When she sends him a letter asking for his help, telling him she loves him, when she first gets herself arrested by the Faith, he burns the letter without replying. The show has made him a much less interesting character as it stands right now. Book Jaime has become a really very likeable character, who has done some bad things in his past and is pretty fucked up, but has essentially come out a good guy. Show Jaime... I don't quite know. After losing his hand and his time with Brienne he became much more sympathetic and likeable... Yet since getting back to Cersei, he's apparently still devoted to her and kind of going back to where he started. They had a great character development on their hands, then kind of forgot about it and let it slip. Hopefully next season we will see a big division between them at last.