1. I don't remember the exact episode, but after dinner with King Elin and his mother, Elis had a fit where he could choose whose vision he would see: Lia/Cass/Elin. I'm sticking with the idea that they are snakes, but then again, things could be completely different.
2. I'm no biology expert, but the cousins seem to be closer to each other genetically. But then again, I'm no expert.
3. Lia scenes are the same whether you went to the ruins or not, the only difference is "whether you played it safe or not." Although, when examined by a doctor, being a bookworm, she could get corr points, but they did not affect her path in any way, but simply changed the scene itself.