Allison reached down and gently closed her hand around her tiny stepbrother, and that very act caused him to shrink to only an inch tall. She had slyly positioned one of her finger to press against Steve's groin, which distracted him from the concentration necessary to regain control of his size.
As she opened her hand and looked down on his tiny, naked body, she said in mock dismay, "Oh, did I do that? I guess I'd better put you somewhere safe so you can relax. Yes, somewhere safe, and warm, and dark, and wet..."
Steve knew exactly what Allison was implying, and had known that this was coming; with Allison's libido and her knowledge of how to control his size, he had been fairly sure that he, sooner or later, would wind up inside his stepsister's body. In fact, she had essentially told him so just the night before:
"You know, Steve," she had said, "since we're not blood related, it's really not incest if we... do things with each other. And I think it'd be pretty cool to be able to walk around with a tiny man inside me without anyone knowing."
"Well, I'd know!" Steve said. "Besides, it'd take way too long for me to shrink so small; someone would notice!"
"Not really," said Allison. "Turns out that sneaking half a sleeping pill into your drink at dinner keeps you asleep but responsive at night. You might have wondered why you've woken up naked in the morning the past few days; it's because your pajamas don't fit you real well when you're only a couple of inches tall. In fact, I really considered getting you smaller and putting you up into me last night, but I didn't want to do that without you knowing."
Well, Steve thought, I guess now I know where I'm going.