Teran looks over the ray gun, noticing the shrink and grow settings, before tilting her head back down to look at your tiny form, which only reaches her knees. "A ray that can shrink or grow people..." she mutters, amazed. "This is so cool!" she squeals, clearly very excited. Your grin widens. "Hey, what are you smiling about, little brother? This is awesome... I've always wanted to be the one in charge!" Your grin falters.
You and Teran have a pretty good relationship - you get on well with each other, and don't fight too often. But then, you are her big brother, so you take plenty of opportunities to boss her around and tease her. Teran slowly begins to smile, and that smile makes you just a little nervous. "Yeah, this is cool... my big brother is now my teeny, weeny, tiny brother!" She leans over to you, her giant 20 foot frame towering above you. You take a nervous, shuffling step back. "You look about a foot and half tall to me. You're so cute at this size! A teeny little kid brother..." You take another step back. "But let's not forget all the times you bossed me around and called me names and teased me... I think this turn-around is going to be fun, don't you agree, little brother?"
Uh oh, you think. It's clear this little prank isn't going as well as you'd hoped. Suddenly, you turn and sprint away. Not the smartest plan, you realize, but you're short on ideas. Teran, being four times your size, easily runs after you, before giving you a shove on the back. You tumble forward onto the carpet. As you turn onto your back, you see Teran's gigantic butt zooming toward you. You yelp out in surprise as Teran suddenly sits right on your stomach. She's really big compared to you, and heavy too, but thankfully she isn't putting her full weight on you.
"Now now, little brother," she says, again relishing the word 'little', "don't think you can run away!" She stands up, allowing you to get your breath back, but before you can stand, she hooks her hands under your armpits and yanks you up. She pulls you up, past her waist and past her chest, until you're eye level with her. You really do feel like a little kid - her face is just so much bigger than yours. "I'll have to teach you to be a good little boy. Good little boys don't run away from big bad Teran!"
With that, she plonks you back on the ground, and reaches for the ray gun. Alarm bells are going off in your head. "It's time for my little brother to become even littler!" she says, grinning wide. Before you can react, she zaps you, reducing your height by half to 3 inches tall.
You can only watch in stunned awe as your big little sister becomes your giant little sister. To your perspective, it looks like Teran just grew from a large 20 feet tall to a massive 40 foot tall giantess. When you shrunk the first time, you barely got a good look at your giant sister or mother. Now, your eyes rise in amazement, following her huge, slender legs up to her waist, unable to help but stare at her huge breasts beyond which you can barely see her face.
"Wow," your huge little sister says breathlessly, staring down at her tiny brother. "This is going to be fun."