Jon breaking his vows to save arya vs rob not exchanging his sisters with jaime because 'they're just girls"
Looking at this another way - Robb doing his duty to the thousands of men who sacrificed to follow him as KITN and not being selfish about his personal concerns and Jon forsaking his duty to the NW and the impending threat and putting his love for Arya above the needs of the realm and his duty to protect it.
As much as we dislike it, Jaime was indeed a more valuable hostage in patriarchal Westeros and because the Lannisters put a lot of value on him - he was heir to Casterly Rock. As one of the strategic commanders for the Lannisters, Addam Marbrand puts it -
Ser Addam snorted disdainfully. “He would have to be an utter ass to trade Jaime Lannister’s life for two girls." - Tyrion, AGoT
As the mother who dearly loved Sansa and Arya puts it -
"He told you true." She could not even say that Robb was wrong. Arya and Sansa were children. The Kingslayer, alive and free, was as dangerous as any man in the realm. That road led nowhere. "Did you see my girls? Are they treated well?" -ACOK, Catelyn VI
Look at it this way - Would the Red Wedding happen if Jaime was still a hostage of the Starks? Would Tywin have risked it if Jaime would be murdered in retaliation? Remember Tywin only considered Jaime to be his heir and not Tyrion. Would the plans now succeed because of the Karstarks not abandoning Robb?
Im not denying the fact that Jaime was a valuable hostage, and i understand that young girls are not seen as significant enough to trade for the "most dangerous man in the realm". But, i think its very telling that rob thinks that he “might have been able to trade the Kingslayer for Father, but . . .. . . but not for the girls?”
He was able to break a betrothal pact from the Freys despite the fact it could have well and truly effected his war campaign (which it eventually did with the red wedding and losing alot of the Freys soldiers) for the sake of jeyne westerlings honour. I just find it sad that he didnt do the same for his sisters
