Tywin can practically taste victory. Having heard about Robb Stark's lessened troop count in the wake of Karstark's execution, Daddy Lannister calls offspring Cersei and Tyrion into his office for some housekeeping discussion. Now that Robb's regime is in shambles, Tywin wants to ensure that he takes every available measure to exhibit dominion over the Stark family; he's also aware that Robb's younger sister, redhead Sansa, is still living under his control, and that Littlefinger is plotting to get her out of King's Landing and into his arms, so to speak. And Tywin can't allow that to happen.
His proposal, or, rather, orders: Tyrion must marry Sansa in order to keep her in the Lannisters' clutches. The imp, naturally, objects to this—aside from the fact that he's currently dating Sansa's maiden, Tyrion has always felt bad for the Stark girl, dating back to her time spent being King Joffrey's subservient, scared, and emotionally tormented bride-to-be. He knows that Sansa deserves better than a hard-drinking, whore-sexing scoundrel like him. Tywin, however, couldn't give a damn less about what either Sansa or Tyrion wants. They're getting married, one way or another.
Which, of course, pleases the smug Cersei, who flashes her sibling a real shit-eating grin upon hearing the news. But then Tywin drops a bomb on Cersei: She's to marry Loras Tyrell, the secretly gay brother of Joffrey's future queen, Margaery, whom Sansa has been fancying all season. No longer all cocksure, Cersei pleads with Tywin like a scared little girl: "Father, don't make me do it again," referring to her undesired first marriage to the late Robert Baratheon. Tywin shuts her complaining down by bringing up her incestuous trysts with his other son, good ol' Jaime. Thus confirming one of Game of Thrones's facts of life: Tywin is a bad, bad man, but also one hell of an authority figure.