Katie starting from scratch / Simon, Coco

When Katie spends time with her mother's boyfriend's daughter, possibly her new stepsister, she learns that sibling rivalry is not sweet.

"Katie's mom asks Katie for a huge favor: will she spend some time with Emily, her boyfriend's daughter? Katie agrees and brings Emily to a Cupcake Club meeting. Things go well--almost too well. Everyone likes Emily so much, Katie wonders if her friends like Emily more than they like her! And if her mom marries Emily's dad, will Emily become her favorite daughter? Katie is an only child, so all this sibling stuff is new to her. She's starting from scratch!"--Publisher's web site.

When Katie brings her maybe-someday stepsister to Cupcake Club, she gets her first taste of sibling rivalry--and it's not very sweet!.