So, the game files allow one to define an NPC's relationship to another. It's as you'd expect -- you can make this NPC that NPC's brother or father or friend, and can even set them as "courting".

So, just a few ones that I found interesting.

-Astrid and Cicero are actually "allies", which is odd considering their opinions of each other. "Allies" simply mean that they'll support each other in combat.
-Jaree-Ra, Deeja, and Gulum-Ei are all siblings.
-Bryling and Falk Firebeard are in a relationship.
-I'm pretty sure that this is a glitch, but Scouts-Many-Marshes and Ulfric Stormcloak are friends (anyone had Ulfric turn up at Scouts' wedding?).
-Galmar and Rolff are brothers; now it makes more sense why Rolff is essential -- attacking him turns Galmar hostile and in turn upsets Ulfric and the rest of his courtiers. Imagine what would happen if he were killed...
-Maven is Ingun and Sibbi's grandmother; Hemming is Maven's son and the father of Ingun and Sibbi. Hemming correctly refers to this, but everyone else (including Maven herself) believes Sibbi and Ingun to be her children.
-Sondas Drenim and Derkeethus are friends; in fact, similar to Mjoll and Aerin, the death of one prevents interaction with the other. If Derkeethus is married, Sondas will attend the wedding, and vice-versa.

These relationships can be checked in-game with "GetRelationshipRank", but they don't show up as more complex than "Friend", "Ally", "Confidant", and "Lover", whereas they are more precise in the game files.

dude, that is awesome info. Thanks for sharing. now I wonder what other stuff is in the game files.....