Erika was led into what used to be Sarah's room by said sister. When Erika entered, she discovered it had turned into a nursery. Erika also noticed that her sister looked a little taller or maybe, more likely, she herself was a little shorter. For all Erika knew, she was turning into a real baby.
'I'm wearing a onesie and a stinky diaper, so maybe turning into a little kid would be better than staying 12,' Erika thought as she was walked toward the back of the room, where there was, among other things, two pieces of furniture she knew were in her immediate future- a crib, and -- Erika shuddered at the thought -- a changing table.
While contemplating all of this, Erika had completely failed to notice that her sister now practically towered over her. She finally realized it as her sister easily scooped her up and laid her down on the table.
Erika blushed a deep red as her sister unbuttoned her onesie- which like the diaper had shrunken with the rest of her- and set it aside before getting to the crappy nappy underneath. Erika's ankles were grabbed with Sarah's left hand- she had gotten young enough for this to easily be done- and her diaper was untaped and opened. Erika was understandably embarrassed that her yonger-now-older sister not only was seeing her naked but was about to start wiping her feces-covered bottom as well.
Erika at first scowled at her sister, but the second a baby-wipe touched her, she began to giggle. She wasn't enjoying this by any means: Sarah was talking to her in stupid gibberish, and the mess being wiped off of her butt had smelled horrible enough inside the diaper but was even worse with nothing to help mask the stench, but that didn't change or hide the fact that she was ticklish.
After this, it was nap time...