It might not have seemed like it, but Peter was actually quite observant. Especially when it came to his sister.
In the whole wide world, Ariel was the person that he had known the longest. For him to not notice things about her, that would have had to mean that he was especially dense. And since Peter was not dumb, he knew things about his sister. He paid attention. Paying attention meant that when Ariel was happy beyond belief, even if she did not say a single word about it, Peter was aware, and her happiness made him feel good as well. But it was not just the happy emotions that Peter was able to sense. When Ariel was hurting and was doing her best to hide the fact that she was upset from him, Peter was normally very good at picking it up.
