Everything you need to understand or teach It's an Aardvark-Eat-Turtle World by Paula Danziger.
More than anything else, fourteen-year-old Rosie Wilson, biracial daughter of a mixed marriage, wants to be part of a family, all living happily under one roof. At first it seems she will get her wish when her mother and the father of Rosie's best friend fall in love and move in together, taking Rosie and Phoebe with them.
The euphoric fantasy of a "happily ever after" family and the ceremony of commitment they make to each other lasts only one day, until Mindy admonishes Phoebe about her public display of affection (making out in a parked car in front of the house with her boyfriend). Phoebe explodes in anger and stops talking to the rest of the family, except for Rosie, who is the only family member to whom everyone is talking.
Rosie's idealistic view of what she wants a family to be is taxed as she is...