Carrie is Cassidy, so...

"We don't put her anywhere," you said. "You've got her body now. I don't see this being a short-term thing, I see this as a sort of... rest of her life thing."

Carrie was shocked. "Are you serious?"

Gravely you nodded, knowing this might be the straw that breaks the camel's back.

But then Carrie just livened up even more. "You do not disappoint, Dave. I thought you'd just want some light bondage, or to get me pregnant. This is... wew. It's like a high I never knew I could get. Am I dreaming? Pinch me."

You pinched Carrie's arm - her sister's arm, but you know what you mean.

"This is real. You want me to be my own sister. And she's just... what? Gone?"

"I know you love her," you said, feeling conflicted. "If you really want... I don't know, maybe we find another way."

But Carrie puts a finger to your full lips. "No. I love Cassidy, really, and it sucks to lose her, but I promised I'd do anything you wanted. Taking her place for you - being her for you - this is so, so incredible."

"Are you serious?" you asked, in disbelief now yourself.

"I love my sister, but I love you way more," said Carrie. "If you want her gone, if that makes you happy, then it's what I want too. But as long as she's around there'll always be a chance she comes back, that she gets her body back, so..."

"So what?" you asked.

"Do you want to go all the way?" asked Carrie, looking to your computer and seeing the file labeled CARRIE'S SISTER on your desktop.

The longer you kept this up, being in Carrie's body, and Carrie being in her sister's body, the harder it would be if you ever wanted to change things back. It was either turn back now, or never turn back at all.