Zack had been amused by seeing Susan, now Susie, sucking her thumb like a baby all day. But in the back of his mind, he felt that it wasn't quite humiliating enough for her. So as he blew out the candles on his cake, he decided to make a rather odd wish. Partially to see how far he could push his new power, but mostly just for the hell of it, since at this point Zack was pretty sure there wasn't anything he couldn't make happen with just a wish.
"So what did you wish for, sweetie?" his mom asked with a smile.
"If I told you that, it wouldn't come true," Zack replied coyly, turning his attention towards his sister, who had stopped sucking her thumb entirely at this point.
"So are we going to have the cake soon?" Susie asked, eyeing the cake intently now that the candles were blown out. Zack guessed that with her babyish interest came a renewed love of all things sweet.
"In a moment, dear," their mother answered, walking towards the kitchen counter, "Just let me get something to cut it with."
Susie huffed and slumped forward in her high chair, clearly getting bored with all of the waiting she was having to do. She rested her head in her left hand, but Zack was more interested in what her other hand was up to.
After a moment, Susie noticed Zack staring and glared at him, "What are you looking at, dweeb?"
"You're doing it again," Zack said, stifling a chuckle as he gestured towards the hand that Susie currently had up the bottom of her dress.
Susie looked down, and her face turned beet red as she pulled her thumb out of her vagina, making a wet slurping noise in the process.
"Oh Susie, really?" Zack's mom scolded as she came back to the table, knife in hand, "You should know better than to do that at the dinner table."
"I'm sorry!" Susie stammered, wiping the excess cum onto the hem of her dress, "I didn't even realize I was doing it! I just got distracted, and it feels weird when I don't have something in there."
"I know it does, honey," her mom said in exasperation as she cut the cake, "But you really need to break that habit. It isn't sanitary, especially with the problems you have making it to the bathroom."
"Mooooooom!" Susie groaned, her face growing even redder in embarrassment, "Not in front of Zack!"
"Hey, it's cool, I already know all about your suckling slit," Zack said between fits of chuckles. Which was true: after all, he was the one that wished her vagina had a suckling reflex. He even made sure it actually had the muscles to constantly be sucking on things, which he guessed made it a little more uncomfortable for Susie to be sitting there with it empty like that.
"Zachary, be nice," his mother chided as she handed him a slice of birthday cake, before placing a smaller piece in front of Susie.
"Sorry," Zack said with a grin. He took a bite of his cake, and noticed that Susie was already starting to quietly slip her right hand back under her dress as their mom placed a noticeably smaller piece of cake in front of her.
"Need some help with that?" Zack said, causing Susie to jump in her seat. After a second, she nodded, and a second after that, she shuddered as her thumb made its way back into her pussy. Zack simply smiled, letting the embarrassment pile onto his sister while he fed her tiny pieces of cake and idly wondered what he should wish for next.