It doesn't. Well, sort of.

You stroked her between her legs. She was burning up. "You're going to live Cassidy's life to the fullest for her," you say. "Get her through college. Keep her friends. Make new ones. Fall in love, out of love. Start a career, build a family. I don't care. Just do whatever Cassidy would've done. You can put your own spin on it, but nobody can know."

The new Cassidy, AKA your girlfriend in her sister's body, trembled and rocked her hips against the pressure of your fingers. "Just like that? I... live her life? My life?"

"No," you said, leaning in and kissing Carrie on the mouth. "I expect you -" you kissed her again "-to keep your promise-" another kiss "-to do anything I ask, when I ask."

Carrie trembled again. "I will. Anything you want."

"You're my baby sister now, Cassidy," you say, smiling close to her face with love.

Cassidy nodded. "And... you're my older sister. Carrie."

"Cass?" you asked.

She looked at you with big, hopeful eyes.

"Be my girlfriend, when you're in town?" you asked.

Cassidy hugged you tight. "Course I will."

You hugged her back.

Then your little sister broke off. "What about my - our mom? I know I can pass for Cassidy, but you..."

You gave her question only a second's thought. You had inklings of what to do about her, but this was moving a lot quicker than you expected.