Mandy Miller wants to join the Unicorns, but the unicorns don't want Mandy Miller! She wears wild thrift-shop outfits, and the other members think she tries to hard to fit in. Jessica thinks she's funny and nice, and she likes her offbeat way of dressing. The Unicorns have asked Jessica to tell Mandy that she can't join, but Jessica just can't bring herseld to break the bad news.
How can Jessica be friends with the Unicorns and Mandy at the same time? Suddenly Mandy gets some frightning news that make the Unicorns seem unimportant to both Mandy and Jessica - and now Jessica is afraid of losing her new found friend for good!

"You mean, I'm not good enough to be a Unicorn?" Mandy demanded.
Jessica's eyes widened. "That's not what I said," she protested.
Mandy paid no attention to her. "I may not have as much money as Lila Fowler or as many clothes as Janet Howell," she said through clenched teeth. "And I may not be as pretty as you are or as popular as Ellen Riteman. But that doesn't mean I'm not good enough to be a Unicorn!"
"I didn't say you weren't!" Jessica cried.
"That's what they're saying," Mandy retorted, "and you know it." Her eyes narrowed. "For weeks you guys have used me to run your errands, and now you tell me I'm not good enough for you? What snobs! I wouldn't be a Unicorn if you paid me a million dollars!"

Will Elizabeth and Jessica's wish come true? The Twins Little Sister