Why does my family not believe in me?

I really want to grow up and be involved with the performing arts. I sing, dance and act in school and outside of school, but I can't tell my mam and dad that I want to do that, because they just tell me not to be stupid and its never going to happen, and I need to find something else. So I decided I wanted to be an air hostess if I can't be in performing arts, and they still said that's not good enough. My sister wants to be a forensic scientist, and now they want me to be that, even though they know science is not my best subject. I do quite well in school, but I feel like if I don't do really well, then they'll think am stupid, and I don't want that. I have a maths GCSE on monday, and I'm scared of failing, not because I don't want to, but because I'm scared of what my parents will say. I took module one and got a B, but I'm not so confident on this one, and even though I'm taking it two years early, I still feel like I can't fail. For my options I said I wanted to take music drama dance and geography but they said I wasn't allowed to because its not very academic, so I ended up doing french, dance, drama & history. Why can't they just trust me and let me live my life the way I want to? And why do I feel like I have to be amazing in school? My older sister did really well on her GCSE's, and I think they want me to be like that, which is really annoying. What should I do?