Robb's Heir?

[David Selig]

If proven, then Jon is the son of Lyanna, the younger sister of Eddard, so as a male, he comes first before Sansa and Arya.

Inheritance doesn't work that way in Westeros.

[Addam of Hull]

If proven, then Jon is the son of Lyanna, the younger sister of Eddard, so as a male, he comes first before Sansa and Arya.

Written with clear premise that Sansa is married to Tyrion and the only reason she will not inherit the North is because of her association with the Lannisters. I personally believe that the Northeners don't really prefer Sansa over Jon or vice-versa, what they follow is the rights of succession, if Bran is proven to be alive, I believe there's no doubt that the Northeners would hail Bran as their ruler despite Robb's will because they know that the will was written under impression that Bran, Rickon, Sansa and Arya cannot inherit due to a lot of reasons. Sansa is no longer associated with Tyrion and the Northeners will definitely see through this.

Cousins don't usually come before sisters, though, with the exception of House Targaryen. The line of succession usually runs as close as it can to whoever the Lord is, which is why children inherit first, siblings second, and other relatives third.

[Spartan64Destiny]

A lot of people I see posting seem sure that Robb's heir is Jon Snow. I don't recall any where in the series where this is stated? Is there evidence to back this up or is this an assumption? Any help?

Read Yeade's essay, "The Grand Northern Conspiracy." Just like the Jon Snow Theory, it is one of the more complex and very thought out Theoretical Essays regarding Rob Stark's will and testament legitimizing Jon to finally carry The Stark name & making him KITN.