Corin was torn. On one hand, this letter was for both his siblings. But, it had taken so long to get Riku to actually sit still and START to listen, he was worried that if they stated, he wouldn't be able to get him to begin again after they retrieved Namine.
But, it was really Riku's call.
Riku was silent for a moment, thinking as he leaned heavily against the prince. "I would not want her to feel, left out, especially from something like that is that is so close to her heart as well."