Who is Jillian Mastrano from Bloodlines and what is their importance?

Jillian Mastrano is Vasilia's half-sister. She is fifteen-years-old and illegitimate. Sydney's assignment is to protect Jill. While the girls are living in Palm Springs, Jill and Sydney are sisters and roommates. Damien is their brother at the school, Keith is their guardian. Kieth lives off campus.