It was a print novel, in English. I barely remember the details. There were three families besides the family of the main character, whose last names were Porta, Gates, and Janus. The main character was a girl, either in junior high or high school, who might have been named Bianca. All I can remember of the plot (don't take this for granted, though: I have a terrible memory) is that she found some sort of egg which stuck itself on her arm and fused with her.
I would be grateful if someone could tell me the title or author.