You feel a tingling, and then a rush of cold, fresh air as you find yourself back in the outside world. You quickly realise that you're on the computer desk in Helen's bedroom, and facing the wall.

You turn around to find Helen sitting completely naked on her armless office chair. Despite your best efforts, and the fact she's your sister, you can't help looking down at her body. Her eighteen year old body is truly awesome, and it's made even more so with her being so gigantic.
"Like what you see?", she asks with a smile.
Feeling somewhat embarrassed, you look away.
"Oh don't bother," she says with a dismissive wave of her hand. "You're going to become intimately acquainted with certain parts of my body.
"What's that supposed to mean?" you, though you have a sickening feeling that you already now the answer.
"I've been doing some thinking while I was carrying you around in my stomach", she says. "And I've decided that I'm going to let you stay small forever. And I do, literally mean forever. I've also decided that you...are going to spend forever...serving me as a slave". She grins evilly and then says, "As of now, you are officially a living sex toy. I am going to use you for my pleasure in every way I can think of. You...will obey my every command. Because if you don't...you will watch me shrink, cook and then eat...mommy and daddy dearest. And then I will torture you. You may NOT speak unless I give you permission. Do you understand?"
You look at your giant sister in complete shock and disbelief. You want to scream abuse at her, but you've been struck dumb.
"Y'know," Helen says with a triumphant smile. "I like you like this. So small and helpless. And quiet for a change. Aaaah, the taste of victory is sweet. Don't worry...slave, I'll treat you well. I may even be nice to you. But from now on, you exist only to serve me. You have no rights. No freedom. No hope of ever being set free. You are a slave. My slave. When I allow you to speak, you will addess me as, 'Mistress'. I've already created an exact duplicate of you, so you won't be missed. You're going to be my naughty little secret. Actually, if you disobey me, I won't just eat mom and dad. No. I will kill your asshole little friends and their families, in the slowest, most painful ways I can think of. And you'll watch. Do you understand?"
You stare at her for a few seconds, feeling totally defeated, then you nod your head.
"Say it, slave" she says softly.
"I...understand...mistress".
"Good. Now, I think you'd like to begin your service by offering yourself to me as a dildo. Wouldn't you slave?".
You have to choke back the tears and the humiliation you feel is completely soul destroying.
"Y..yes mistress", you reply meekly. "I would...like to...offer my body to you. To use as..as..a d..d..dildo".
Helen chuckles, "The taste of victory is so very sweet", she says as she licks her lips and starts to rub her pussy with her right hand. "I'll just make myself nice'n wet, so you'll slide in easily. Watch me".
You watch helplessly as Helen spends the next ten minutes, playing with he pussy. You can her becoming increasingly aroused, panting breathlessly, groaning softly as she rubs her pussy with one hand and slips a couple of fingers inside with the other hand. It's pretty obvious that she has a couple of minor orgasms. Suddenly, Helen opens her eyes and looks at you. She smiles and hungrily licks her lips. Now it's time to put you to work. My little living fuck toy."
Helen's eye's glow blood red, and then you grow to ten inches. It's only then that you realise, that you too are completely naked.
Helen lets out a soft growl as she reaches for and then: