[@AtomicNut] Well the Karstarks and House Thenn probably don't get on too well because of the history there. Harrion and Alys are likely amiable but their children might have some kind of rivalry because of competing claims and the wildling/northerner enmity. 

Or maybe House Thenn did get a claim to Karhold if Harrion died, but the other Northerners simply would not have a wildling house control such an important holding and lobbied for the other Karstarks to be given the land, to which the Starks acquiesced.