Miranda explains that her parents got divorced the summer before ninth grade. Now, Miranda sees her dad very little, and her mom doesn't talk to her much. Miranda didn't want to go to camp over the summer, but her mom insisted she go. For the most part, it was awful. Miranda didn't know anyone and to cope, she started making things up about her family. She even said her little brother was deformed. This prompted a dramatic reaction and Miranda felt terrible—she knew that Via would be upset, but she also felt somewhat entitled since August feels like her little brother.
Like Justin, Miranda also feels isolated and unsupported in her nuclear family. When she starts talking about the Pullmans as though they're her own family, it suggests that she recognizes that the Pullmans are infinitely more supportive than her true family—and in turn, that she recognizes that she absolutely needs that kind of support and intimacy with her family.
The lies turned out to be good for Miranda's popularity. Suddenly, everyone wanted to hang out with her. The popular girls cut and dyed her hair, helped her with makeup, and told her how to dress. They smoked and hung out with boys. When Miranda got back from camp, she contacted Ella and not Via because Ella didn't ask hard questions.
By changing Miranda's outward-facing identity, the popular girls were able to make Miranda look like someone who belongs in the popular group. Miranda's unwillingness to contact Via suggests she's not entirely on board with these changes.