He's looking down at his seven-year-old sister, trying to decide what to do. She's trying to get up, but he's holding her down too well. As he's thinking, he notices something with what she was wearing. (A diaper? Hmm... I don't know what to do with her properly yet, but I've got an idea.)
He picked up his sister again, and pulled his pants down, revealing his semi-hard cock. He grabs the diaper off the bed, and steps into it, pulling it up; It's tight, being made for a five year old, but he can make it work. Sitting down on her bed, he pulls the waistband out, and shoves her head-first into it.
He pushes her legs down into her former diaper, closing it back up. He can feel her moving around, and combined with the size she's pressed up against his body. While he can't quite tell, he's pretty sure his penis is pressed up against her opening (Luckily it's not inside), and he can feel muffled screams near his butt.
With it being small for him, alongside having to contain his sister, it's being stretched pretty far. He stands back up; His legs are forced to the sides, and an entire body is held between his legs, but he can still somewhat walk. (This is going to be weird to get used to, but it's only temporary, until I can think of something else.)