Scurvey, Lumpy, and CS, I welcome you all to the Chronicles Network. We love your questions and ideas. I'm gonna throw out some of facts and ideas that have been tossed around here before... let me remind you that you do not have to like these ideas or accept previous hypotheses as facts. By all means, we would all love for anyone to come up with better hypotheses or find clues that no one else has found.
Brynden Tully, aka The Blackfish, was Robb's leading battle commander and his castellan at Riverrun. Robb did not have a Hand, but Brynden did a lot of a Hand's work even if he never had the title. If anyone knows what Robb decided regarding the inheritance of the North and if anyone has the determination to see Robb's will done, it is The Blackfish. In addition to Jon and Sansa, Robb's blood relatives included Brynden and Edmure. Don't forget that Jaime failed to capture Brynden... he's on the loose.
Regarding Jeyne Stark nee Westerling... In ASOS, Catelyn noted that Jeyne's hips looked ample for birthing. In AFFC, Jaime noted that Jeyne seemed slim of waist. Did Jeyne hit a growth spurt? Or did her mother try to fool Jaime with a stand in?
Edit: My apologies, I had to run out for a few minutes.
Mayhaps Lady Sybelle is still playing both sides... I'd certainly like to think Robb has an heir of his body.