The unique spiritual bond between a mother and her sons, and the psychic damage to both when that bond is severed.
This is the central theme/motif of Rhaegar’s prophecy, perhaps of the series itself.
After all, what are the Others but a supernatural army of abandoned sons? Maybe they just have serious attachment issues.
In addition to establishing the above point, I believe I can strongly argue the following:
Rhaegar’s song (“Jenny’s song”) has a subtext that only women can perceive, concerning the loss of children–perhaps sons in particular.
The known line of Jenny’s song is actually a refrain, appearing multiple times.
This allows for some interesting speculations about the origins of certain characters.
Sansa is uniquely suited for the task of invoking prophecy against the Others if necessary.