I'd like to think that Jaime is the valonqar and will ultimately be Cersei's undoing. Cersei has spent so much time and effort hating on Tyrion and judging him inaccurately time and again with only bias to back her judgment up, she's blind to any other possibilities. While she and Jaime are twins, she was technically born first, which technically makes Jaime the younger brother, even if only by minutes/seconds.
Jaime's been manipulated, used and abused by his sister/lover for most of his life, and he's only just (in SoS) started to realize to what extent. He has a thought, I forget where exactly, that's "She never comes to me, she waits for me to go to her," and I feel that's significant because he's becoming aware of the dynamics of their relationship. When Cersei doesn't get what she wants from Jaime, she puts him down, tries to emasculate him, which is a really crap thing to do, especially to the "love of your life."
Likewise, we learn through Cersei's POV in AFFC that while Jaime has only ever wanted Cersei, Cersei was all about Rhaegar, and it was a devastating blow how that all fell out. Jaime was her twin, was always there, her second choice, and infinitely better than anyone else, seeing as Rhaegar was now unattainable and soon-to-be dead. Furthermore, she makes it clear that her "love" for Jaime is 100% conditional. If he does what she wants, she loves him and "rewards" him. If he does not do what she wants, she viciously attacks him, mostly verbally, but occasionally physically.
Does Jaime have reason to want her dead? That depends on Jaime's character. Has he realized what Cersei is? Yes. He knows she's screwing/screwed the Kettleblacks and Lancel, and when she sends her "I need you now as I have never needed you before. Come at once. I love you I love you I love you" letter, he reads it over and chucks it into the fire. He's no longer going to "hop-to" just because she tells him so.
But does Jaime have it in him to kill her? It's hard to say. Jaime loves his sister, and he has been in love with Cersei for all of his adult life. Despite "breaking up with his girlfriend" (for lack of a better term), he still loves his sister, and I certainly don't think that he's at a point that he would just kill her. Does it mean she wouldn't find a way to escalate it to that? No. But Cersei seems extremely cowed after getting sheered and made to walk naked through KL. Does she have it in her to rev Jaime into murderous rage again?
I think that Jaime killing Cersei would be poetic, both because he is also, technically, a younger brother, and because he's been really burned by her, but Tyrion has far greater reason to want to kill Cersei. She's done every single thing she possibly could to ruin/end his life. She's blamed him for murdering her son because she's so biased against him she was unwilling to examine any other possibility. She tried to have Mandon Moore kill him because he was undermining her. She has wronged him more grievously than she's wronged any other person, bar none, and that is saying something.
What we do know of Tyrion is that he's on his way to joining forces with "another queen, younger and more beautiful, that will take all that you hold dear." He's flat out stated that what he has to offer Queen Daenerys is all the information that he possesses, which is pretty much everything. He intends to give Dany the rub on all the Lannisters, the wars, the ins and outs of KL and the current "ruling" system in Westeros. The thing fueling Tyrion is vengeance. He wants his brother, and especially his sister, to pay, and pay dearly. Like Arya's prayers, Tyrion wraps himself in the warmth of his hatred and determination to destroy them.
I don't believe that Jaime (at this point) has it in him to kill his sister, the woman he once loved, not without something huge to incite that kind of rage, and Cersei (at present) doesn't seem to have it in her to induce much of anything in anyone.
I wholeheartedly believe that Tyrion is the valonqar to bring down Cersei, in one manner or another.
