Edmund's eyebrows raise up. He knows that Caspian and Peter don't always see eye to eye, but he's never heard Caspian say something that directly insulted the High King. But there's more said in the action; Edmund can tell that Caspian is frustrated, and worried, and Ed has more than enough experience to not feed that frustration.

No matter how much he agrees with Caspian at the moment.

"What was he even doing?" Ed asks, genuinely curious. Why would Peter visit a graveyard anyway? It wasn't exactly a cheerful subject, and while his older brother is serious he's most certainly not morbid.