As DDFirst Name Dd stated, I believe that Catelyn stated that Eddards Grand-Aunt married into another house, which I believe is house Royce, although I may be wrong about this. If for whatever reason these distant relatives are ineligible to inherit, such as dying without issue, then there would likely be another house who can trace descent through a grandparent or great grandparent to a direct male-line ancestor of Robb. Unfortunately, it would seem that Westeros, or at least the north, does not have a directly clear system for dealing with succession, and as such the claimant with the most direct blood relation to the current Stark's of Winterfell may not be the best potential claimant. In Westeros there are numerous examples of claimants with lesser claims being favored over the most direct heir, such as the first Lord Paramount of the Reach, who was granted dominion over Highgarden and the Reach despite the fact that while numerous Reachman houses claimed direct descent to the Gardeners through the male line, such as the houses of Oakheart, Redwyne, Fossoway, Oakenshield, Tarly, and many others. This is because in Westerosi culture the ultimate authority is the King. If a potential claimant has no real chance of holding said title, it would be unlikely that the current king would appoint another person to said title. As Robb is the King in the North, so long as that kingdom continues to exist, his designated heir, Jon in the books, if the will of Robb had been read, and only if Jon was willing to forsake his vows, and the other northern houses were willing to accept his as their king, then he would become the new Lord of Winterfell. However practicality would likely dictate that another, such as Lord Karstark, or possibly even a member of house Royce, if they are in fact the house that Catlyn mentions, would step up to claim Winterfell. In the end, the heir to Winterfell is whoever is able to hold it, be it a potential husband to Sansa or Arya, a distant cousin backed by the other northern houses, or even the Karstarks, any could claim legitimacy, and in the end, sucession in Westeros is not about who has the best claim, but who has the most power.