"Hey Gary, what happened to that boy?" Mary asks her brother, looking at his bare butt. She's sure that he was in front of her, but she couldn't see him anymore.
"Hehe, take a wild guess," Gary responds, smacking his buttcheek to send the point home. Mary cringes at her brother before forcefully grabbing his hips and crouching behind him, face-to-butt.
"You're disgusting. I can't believe I have to do this," Mary says. Before Gary even has a chance to ask, he feels a finger enter his ass. It definitely feels different than water (or a squishy boy) entering his butt. It just feels weird. Then he feels another, before a stretching sensation. Mary pulls his butthole open with two fingers, gaping open his colon. Immediately plenty of water flows out, already smelling not great from having been up his butt. Luckily there wasn't any poop, a fact Mary thanked the heavens for. She expected you to just slide right out, but it doesn't happen.
"Great. You deserve this, by the way," Mary says before forcing her entire hand up her brother's ass. He moans from the intrusion before clenching his buttocks around her arm. "Hey! Don't you clench me!" she says before slapping his buttcheek with her free hand. The discipline makes Gary unclench, letting her delve deeper into his butt.
Suddenly you feel a hand grip you. At first, you thought it was just another clench from Gary just to make you feel worse, but then the grip starts pulling you out, sending you through the bowels to where you first entered. All at once your eyes have to adjust to the bright light of the sun. You fall into a semi-translucent puddle on the ground, slowly and limply stretching upwards and reforming into your original self. As you're reforming, you can still hear and see what's going on well enough.
Mary lets go of you onto the ground and pulls her hand out of her brothers ass, who collapses onto the ground himself in a daze from having his ass stretched to new circumferences. Mary only looks down on her brother with anger and shame.
"You're gross. Don't think I won't rub this in your face for years," Mary says to her brother. Gary fails to respond, still in a daze.
At this point you are back to your original height, and close to a human shape. Mary walks toward you and gently puts her hand on your shoulder. Unfortunately, you notice her turn up her nose, undoubtedly because you smell like the inside of her brothers ass now. You feel like you're about to cry, and Mary clearly notices this and feels bad.
"Hey, hey, listen. It's okay. Let's go to the showers, alright?" Mary comforts you. You shake your head, you definitely like the idea of getting clean.