Is it right for me to go to my sisters senior prom

So I have been dating this kid for about 8 months and last year he was a junior and he asked me to junior prom my parents let me go. I have an older sister who is in the same grade as him. This year he asked me to go to his senior prom but my parents told me no I don't think that's fair it's his senior prom but my parents are saying I should let my sister have her last prom without me. A couple days ago me and her went to a football game together and she even said we had fun at the tailgate and the game so I don't get why my parents won't let me go we have fun together and at junior prom I didn't talk to her the only time I did was when she found me to take a picture together for my grandma. I say this is unfair what do u guys think?