Oh Sansa, Sansa, Sansa. Poor Sansa. She thinks she's being fitted for her lovely new frock, only for Cersei to whip out the wedding cloak at the last minute, and suddenly she is being whisked away to marry a dwarf.
It's the best wedding King's Landing has ever had! It's got a crying bride, a humiliated groom, and a family argument. The highlight of the evening is going to be stripping the bride and groom naked and listening outside the door while they have sex. Because Westeros is oh so civilised compared to those Dothraki or those wildlings...
Tyrion isn't having it, though. Sansa is only 13 and his heart's not in it. He probably is going to be a fairly decent husband. All Sansa has to do now is avoid Joffrey's attempts to use her to make bastards.
Lovely family, the Lannisters.