I, Brian,
It's 2 a.m., I hope this makes some sense.
A relationship between Jon and Sansa might be a far fetched concept, but it is in the realm of political possiblity. Robb declared Jon his heir, even though Jon rejected Stannis' same offer. So, Sansa as the eldest, Bran as the eldest son, Rickon as the only healthy son, Jon as the ******* son with the semi-legal claim, Tyrion Lannister as Sansa's husband, and Ramsay Bolton with the fake Arya (Jeyne Poole) all can claim Winterfell. To keep the Starks in power, the Bolton's out, the Ironmen out, and the Lannister's out... Littlefinger and Sansa will maneuver and plot. Who's to say the Night's Watch won't get involved? The Lannisters seem to think Jon is already meddling in politics.
Since the Starks are the good guys, it seems horrible to think of incest on their part. The Targaryens, Lannisters, and perhaps Tyrells are already doing it... but since they are the bad guys, it's okay.
The probable scenario for Jon and Sansa would be if 1) the Other and Others are defeated permanently, 2) the Wildlings are pacified, 3) there is no need for the Night's Watch, 4) the North has not healed from the wars, 5) the Lannisters or Greyjoys seem to be about to legally take Winterfell, 6) Jon turns out to be Sansa's cousin and not her brother, 7) Jon is declared the legal heir of Robb, 8) Sansa needs to escape the clutches of Littlefinger and/or Tyrion, and 9) the realm needs to be united against the Targaryen threat.
Now, Raven can post and shoot my musings down.