You are the daughter of Lily Evans and Severus Snape. Since Harry Potter was also his mother Lily Evans, he was considered your brother. Severus didn't know about you because he was drunk the night he was with Lily and you were their illegitimate daughter. After Lily and James died, you and Harry were sent to the Dursleys' house. You were mean to Harry because your mother cared more about him and most importantly Harry was different from you and he was a pure blood. .