How to convince my parents to let me get a haircut?

Ok so I'm 13 going to 14 and I really want a haircut. Well, I come from a Sikh family and it is against our culture to cut our hair. My little brother has long hair but my dad an my big brother have haircuts. If I didn't have older siblings I would probably get a haircut because I cut my bangs once an my parents slapped me and my big sister took away my laptop an everything just because I cut my hair. My sister has long hair to and she straightens it and I don't have a straightener. I never really asked my parents that I want a haircut I just cut my bangs once because I was tired and sick of my long hair an I still am. I've wanted a hair cut since I was 7. My hair is up to my knees. So how do I tell him to let me cut my hair?