What is the connection between Mari and Lara in the novel, The Villa?

Lara is Mari's stepsister. Lara's mother marries Mari's father when the girls are both 12 years old. Although they have a period of closeness, Lara quickly becomes competitive of Mari. This dynamic follows them throughout their adult lives.

During the summer of 1974, Lara becomes increasingly frustrated by Mari's behaviors. She accuses Mari of stealing both Pierce and Noel from her. Despite her upset, Lara comes to rely upon Mari after discovering that she is pregnant with Noel's child. She ultimately miscarries the child, and goes off to launch a successful musical career of her own.