I don't know about that. Cat released Jaime in that situation because he was likely going to be killed by Karstark's men that night if she didn't act. Yes, she wants to try and get her daughters back but I don't know that having the Karstarks kill Jaime as a prisoner would help the North's position either.
I have to imagine if that happened and word got back to Tywin, they'd kill off Sansa or make Sansa's life very unpleasant. And what would that have accomplished? Maybe the Karstarks are still on Robb's side, but maybe then Robb ends up killing Karstark as a traitor anyway for killing Jaime, just as he did for killing the Lannister squires. Except now you'd likely have a VERY pissed off and motivated Tywin gunning for you. Whereas currently, Tywin might be more reluctant to act against Robb if he thinks Jaime is in Robb's custody.
I think Cat was just trying to make something out of nothing when she released Jaime. It was just a bad situation all around- either Jaime was going to get killed and they'd get nothing from his death beyond maybe Karstark's vengeance being sated and Robb having to respond to that insubordination when he returned or she could try to actually get her daughters back from King's Landing in a prisoner exchange that might calm tensions with the Starks and Lannisters.