How can i get help from her?

am in love with a girl witch is my sisters best friend. i believe that this girl like me too because she wants to spend too much time with me every time she comes for my sister. but when my sister learned that i like her friend she decided never to give her even a call again and she is doing whatever she can to keep her away. i ve asked many times why she is doing that and she never answers. my sister is the only one who can help me see this girl. (i cant give her a call because her dad gets angry every time a boy calls her) (we are about 14-15 years old)