Why do they only treat me like this?

Me and my older sister live with my grandparents and have since I was in the 4th grade. My sister is 3 years older than me and she has always went to parties and did what she wanted. I have always been good at school , gotten good grades, never really went out not as popular as my sister. My sister did well in school too but she also got to party. I’m 18 I realize that I want to go out more bad have more fun. However every time I try to go somewhere they make a huge deal out of it. My sister used to stay out till about 2 am when she went out. But I tried to come in and change my clothes to go to a party at around 11:30 and they told me I could not go. I am 18 and I turn 19 next month. This is not the first time either. When I was truthful about where I was going they still told me no. So I told them I was going to my other grandmas house for the night and they said me and my sister lie too much about where we go. I Never lie to them about where I am going. I did this time because I could see they were questioning me too much. I got nervous. I never really party st all and I really just started this year. I always do what my grandparents ask but this one time they tell me I can’t leave. It’s not fair. My sister has always down what she wanted so why not me. I’m an adult now and I asked them what the difference was he says are you going to do school or play? I have straight a’s in college what more do they want from me. What should I do?
Why do they only treat me like this?
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