You checked if Sam was home, and she was. She was reading a magazine in complete relax mode. She was happy to see you for some reason though.
"Mel! Did you see these new swimming suits they have at the mall? We should totally go buy some and go swimming tomorrow!" She said.
"I kinda wanted to tell you something, which is why I got here." You said.
"What's up? You look nervous. Hello? It's me, your older sister who you don't need to be nervous in front of!"
"Thanks sis. So you're probably not gonna believe this, but what would you say if I had the ability to swap body parts around?"
"I would laugh right at your face while asking for proof."
That gave you a reason for revenge and some inspiration as you looked at her and thought about what to swap. Thinking about how her chest has always even been bigger than your ass, you swapped them. The expression on her face was priceless.
"WHY IS MY CHEST SMALLER AND WHY DO I FEEL MUSCLES AND A HOLE?!"
"Relax, that's just the proof you asked for."
"CHANGE IT BACK NOW!"
Because she looked legit frightened, you swapped back, but with a little twist, a twist she'd have to find out herself, because you kept the nipples and let her keep the asshole.
"God, thanks. So that power is real. Should we go outside and fuck with some people?"
"Sure, let's go to..."