How can I get my family to support my dream of becoming an actress?

I'm 15 and I live near Boston Massachusetts. I have wanted to be a famous actress since I was three and my family has never supported me. My sisters always say that I can't act at all and it really hurts. My grandmother has never supported me, she takes it as one big joke. And when I try to talk to my mom about she always says "not right now". I really want to be an actress. Today I saw that Disney Channel was having open casting calls in Virginia and asked my mom if she would take me and my Grandmother said that if I was any good I would have been in theater by now. I started to cry and ran to my room. This has been my dream for so long and everyone just laughs in my face about it. Please help me. If you know how to get an agent and how much they will cost, I really want to do this. Please help me...please.