1 Work in Roslin Frey/Bran Stark
Jon knows the men of the Night's Watch may be his brothers by oath, but Robb is his brother by blood. With father dead and Sansa and Arya trapped in King's Landing, he cannot sit idly by at the wall and abandon them.
Robb knows he can't marry one of the Frey's. This is just a small stepping stone on the path of getting his sisters back, and he'll need more allies later if he is to win this war. Instead, he offers Bran, his brother and heir should he die in battle. Turns out a Prince will work just as well as a King.
Sansa knows she cannot possibly stay in King's Landing. They cut her fathers head off and, when she begged to take her supper in her rooms, Joffrey brought it to her on a plate. She waits until night and, with the help of a now very large direwolf, escapes before the Lannisters know she's gone.