Something that I just stumbled over in my re-read of A Storm of Swords concerning Cersei and Tyrion.
' Willas Tyrell. Tyrion was taking a wicked pleasure in Cersei’s helpless fury. “That would be the cripple,” he said.
Their father chilled him with a look. “Willas is heir to Highgarden, and by all reports a mild and courtly young man, fond of reading books and looking at the stars. He has a passion for breeding animals as well, and owns the finest hounds, hawks, and horses in the Seven Kingdoms.”
A perfect match, mused Tyrion. Cersei also has a passion for breeding. He pitied poor Willas Tyrell, and did not know whether he wanted to laugh at his sister or weep for her.
“The Tyrell heir would be my choice,” Lord Tywin concluded, “but if you would prefer another, I will hear your reasons.”
“That is so very kind of you, Father,” Cersei said with icy courtesy. “It is such a difficult choice you give me. Who would I sooner take to bed, the old squid or the crippled dog boy? I shall need a few days to consider. Do I have your leave to go?”
You are the queen, Tyrion wanted to tell her. He ought to be begging leave of you.
“Go,” their father said. “We shall talk again after you have composed yourself. Remember your duty.”
Cersei swept stiffly from the room, her rage plain to see. Yet in the end she will do as Father bid. She had proved that with Robert. Though there is Jaime to consider. Their brother had been much younger when Cersei wed the first time; he might not acquiesce to a second marriage quite so easily. The unfortunate Willas Tyrell was like to contract a sudden fatal case of sword-through-bowels, which could rather sour the alliance between Highgarden and Casterly Rock. I should say something, but what? Pardon me, Father, but it’s our brother she wants to marry? '
'A Storm of Swords, Tyrion 3, by George R.R. Martin.
So, alright, Tyrion and Cersei clearly have little affection left for each other by this point, since Tyrion thinks Cersei wanted him killed, and Cersei most certainly would have loved to see her little brother keel over for pretty much as long as he lived. I still find it interesting how Tyrion reacts in this chapter. He clearly wants her miserable, and with good reason (and vice versa, Tyrion threatened Tommen, after all), but almost siding with her at other points. And if all their baggage wasn't there, he should! The whole scene, its so plain to see their father is doing the exact same thing to them, pushing them around as pawns. It'd make for an interesting AU to consider how it all would have turned out if Cersei hadn't had reason to hate Tyrion from an early age on. Tyrion is extremely suspectible to affection; he'd probably have been as attached to her as he was to Jaime. And these two in Clash of Kings/Storm of Swords in a position were they weren't constantly trying to undo each other...
The three Lannister siblings united, I feel that would have been a force even Tywin shouldn't have underestimated. They might, with a little luck and much cunning, have been able to somehow wrench away the control he had over them all their lives. Or at least they could have worked on making their lots more bearable - how long would Robert have survived if Tyrion had been his sister's private counsellor at court, deeply invested in the happiness of both his siblings? Would Cersei remorselessly manipulate her little brother to do her bidding since she has him by his one fatal weakness, or could she enjoy having a true friend? And so on... Argh, fanfiction, it calls like a siren.