For as long as Peter had been alive, he had always been a younger sibling. He had always had an older sister. But now he was in very real danger of being somebody who'd 'had a sister', That was a terrifying thought to have.
In the past, whenever Peter had thought of how he would react, in an awful scenario where he had to bury Ariel. Peter had always thought that it would be a simple matter of him falling apart on the spot, and not being able to move anymore.
But now the very situation that he had always thought of as an abstract concept, had a very real chance of becoming reality. And what Peter had thought would be true was instead being proven false. Instead of falling apart while waiting for news on his sister, Peter was a ball of energy. He could not sit still no matter what.