Interesting. I always thought it was because most elves only get one cub. But the relation between siblings is just as important, yes. And it is my guess they only use those terms to first degree? blood. Treestump was brother to Joyleaf so Cutter called him uncle, but I haven't seen Ember or Sunstream call him that. And who knows, maybe Nightfall called Scouter cousin, but we just never read it. Although, I get the feeling she wasn't that close to One-Eye.