It was rough to try and sit still, for Riku. But sitting next to the Prince was better than not being beside him, and not having the comfort of his presence beside him. More, he moved closer to Sora, even if it was further away from his brother, because he knew that his brother wouldn't understand as well. Wouldn't understand his apprehension, his nervousness, and he wanted to be comforted right now, not rebuked.
Plus, if his head was against the Prince's shoulder, he couldn't see his brother's annoyed, frustrated expression. It was their parents! Why was Riku acting like this was a punishment? "Do you want to open it?" Corin asked, and Riku shook his head. No, no he didn't.
"We sent them a letter," Coring reminded him, and Riku's back grew tense.