Your Adventure: On a hunting expedition around Lake Ilinalta, out of a cabin owned by her Uncle Dengeir, during which she slipped away from her various Guards and Handlers... as is her common tactic.. to track a Sabrecat that she'd not been able to corner earlier in the day. A mundane hunting expedition turned chaotic, though, when she was ambushed by a group of bandits and nearly killed.
Crossing Paths: Yvaine was nearby and heard that Dagny's travelling party was nearby, having decided to go and check in on her friend, only to find the ruins of their caravan and most of the people dead. She tracked the Vampires back to their lair... only to find Dagny 'enthralled' by the Undead!
Skyrim: A true daughter of Skyrim, Yvaine has very little patience left for the Imperials and views what they did with her Brother to be-- just /wrong/,-- but, unlike her Uncle, she isn't entirely comfortable with the Stormcloaks either. This is partly because she was there when Ulfric killed Torygg and try as she might... she can only see it as Murder. This leads her to be disillusioned with the entire prospect.