I'm three quarters through the third book, "Storm of Swords" right now (so please, no spoilers from too late in the book or any subsequent books). I just read a part where Tyrion is discussing with Prince Oberyn and they mention the difference between Dornish law and Westerosi law in Line of Succession (ie women can rule under Dornish law).
I'm a little confused trying to puzzle out what the exact LOS rules are.
At first I though it was: at the moment a king/lord dies, to find who holds the title next, you do a depth-first search down the family tree starting with the oldest sibling (ignoring anyone who isn't true-born or has given up their claim ie as Night's Watch or Kingsguard etc.). Then, for Dornish law you take the first living person and for Westerosi law the first living male person found in this way (so succession can pass through a woman in both cases, but only Dornish law actually allows them to claim the title). If there are no valid living heirs, then you work backwards along the paternal line until you find an ancestor who has living heirs and start from them.
But there were various references to if Sansa's brothers were all dead, then her child would claim Winterfell. It seems to me that that's only true if somebody else doesn't get it first. In other words, if Sansa's brothers are killed before she has a child (even if she's already married), doesn't that mean Winterfell would pass to the nearest cousin along her father's line (whomever that may be)? And then because that cousin has already held Winterfell, when they die the next in the line of succession starts from them so Sansa's children's family would not have claim to Winterfell again unless that cousin has no valid heirs.
What am I missing here? Is there some exception? Are there no valid cousins?