Lucy buried her head in his mane to hide from his face. But there must have been magic in his mane. She could feel the lion-strength going into her. Quite suddenly she sat up. “I’m sorry, Aslan,” she said.” “Now you are a lioness,” -Aslan
It's a toss up between this one and Peter/Edmund brotherly relationship or Susan/Lucy sisterly relationship or Peter/Lucy sibling relationship. Sorry Susan/Edmund sibling relationship, it didn't do it for me.