Ask her about the magazine
From Create Your Own Story
"So, sis," you murmur, "what were you doing with a perverted magazine?"
"Um..." Callie mumbles. "I... um..."
"Were you thinking about performing activities like the couples in the magazine?"
Callie's face turns a deep red. She nods, shamefully.
"Tomorrow is the party," you remind her. "Mom, Dad, Katie and Helena will be away. I'll be taking care of you... and if you would like to experiment with something like you saw, we can."
Callie whispers, "You're sweet. I think I will accept your offer."
You put your arm around her. "I love you, sis." At that moment, the door opens, and your mom slips into the room, unnoticed. "I'll make sure I take proper care of you tomorrow. It's my duty as a brother to do so."
"That's exactly right," your mom chirps. "It is your duty to keep her safe and protected tomorrow. I was expecting you to whine about it."
"Nope," you say. "Callie is very sweet and I'm looking forward to keeping her company."
"Good," Mom states firmly.
The next day, as soon as the rest of your family leaves, Callie summons you into her room. She's already stripping when you get there, and you also disrobe quickly. Callie pulls out her magazine and points to a couple of positions. "I'd like to try one of these."
You look and see what she's pointing at.