I am 14 and haven't seen my big sister since I was 7 I desprately want to find her but how can I at the age of 14.

Ask your parents for starters.

My dad doesn't want me to no about her and my mum won't answer any of my questions

Legally you are not entitled to any of this info. You may or may not be able to obtain it when you are of legal age. Why is she not with your family?