Lila's Little Sister #4 - The Unicorn Club Series 
Francine Pascal (1994)

Ellie McMillan, a little girl from the day-care center where the Unicorn Club volunteers, has run away from home—to my house! I know, you're thinking Lila Fowler and kids go together like grape juice and white carpet, but —believe it or not— I actually enjoy reading and playing hide-and-seek with Ellie. It's almost like having a little sister. The only problem is, she wants to move in permanently! You see, Ellie's mom just started going out with a new guy, so Ellie's been spending a lot of time with me. I guess she figures if her mom has a boyfriend, she doesn't need Ellie anymore. I promised Ellie I wouldn't tell any grown-ups that she's at my house. But Jessica and the other Unicorns say Ellie has to go, promise or no promise. Is there a way to keep my word and get Ellie back where she belongs?

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