AU, from the end of 3x07 in which certain events don't happen and Brienne ends up as the reluctant guest of the Lannisters in King's Landing. (TV universe only, though some details from the books might sneak their way across)

Tyrion knew, as he gazed up at the dark skinned Martell man that this was perhaps his only chance to spirit Sansa out of King's Landing and away from his father's greedy fingers. Tywin Lannister would have married the girl himself if he felt he could have gotten away with it, and so Tyrion knew his own marriage was no protection to Ned Stark's daughter whatsoever. "Invite her, demand her, steal her--whatever you choose, Prince Oberyn, choose it soon. Else my lady wife shall break into more pieces than the stars."