Dawn's Wicked Stepsister — Ann M. Martin

Now that Dawn and Mary Anne are friends and sisters, Dawn wants them to do everything together -- share a room, talk all night long, wear each other's clothes.

But living with Mary Anne isn't exactly what Dawn expected. Mary Anne brags about having a date to the school dance, her kitten throws up on the rug, and she hogs Dawn's baby-sitting jobs!

Dawn's always wanted a sister. Instead, she got Mary Anne, the wicked stepsister of Stoneybrook!