Try to think from Caspian's perspective.
Dear authors:
I am not "the fifth Pevensie." Or another brother to the Pevensies. I am not part of their family. I look up to them as the great kings and queens of old.
And you do know that would make Susan and Lucy my sisters, right? Kind of puts a damper on possible romance, doesn't it? Imagine the conversation:
"Dear brothers of mine, I wish to marry your sister."
"Oy, if we're your brothers, she's your sister too!"
"Oh. I hadn't thought of that. I suppose we shall have to have a torrid incestuous affair, then."
"By all means, go ahead. The four of us are always marrying each other anyway, it's nothing new."
"What?"
"What, haven't you seen all the writers who know us so well that they know Edmund and I secretly have a burning passion for each other?"
"Or the ones who insist Lucy and I are meant for each other, and that here in Narnia incest is perfectly moral?"
"..."
"Don't forget the ones who marry Susan off to me, either. It's like they don't know that incest causes genetic defects, or about the Westermarck effect, or that we would all be bloody disgusted by the very idea."
"Or the people who insist Narnia can actually remove the sibling-ness from people, thereby making incest fine and even cute."
"..."
"And then there's always the people who think you and I are soulmates. Or you and Peter."
"..."
"I think you made him sick."
"Me? What did I do?"
"You told him about the perverted fangirls. Now he's throwing up. And he has a council meeting in five minutes."
"Wonderful."
~Caspian the Tenth
P.S. I know what Benjamin Barnes said. He was wrong.
Badfic!Caspian: "I've admired you and looked up to you in stories and legends since I was a little boy. Let's hug, bro! Then I can go marry my sis!"
Sense, it maketh not.