Brothers 21'st birthday party...?

He wants me to go ( seems more like an order than an invite) but there is going to be around 50-60people his own age there all between 18-30. I Don't know anyone but none of my friends can make it because of the short notice, they have things planned. I Don't want to go to it. It's not even on his real birthday but the night before so it's not like we're actually celebrating his birthday on the day. He said theres this 16year old girl going (i'm 15 and a girl) but i don't want to be talking to a stranger all night at a party where there will be loud music and noone could hear me anyway! I Don't want to go and i don't know how to tell him this... i feel upset about going because not only am i going to be on my own but i'm also going to be aongst most of them drinking. It's in a hall and even though my dad will be there it's a party which means talking to people and having fun but when they're like 20 onwards and i'm 15... kinda awkwad! Not to mention i'm awfuly shy! :( I Feel nervous about the party and the only bit i want to get involved with is the help decorating with the ballons. Why do i have to go? It's not like it's my party, it's my brothers and i'm 15 i think i can decide to go or not go to a party you know? But because he's my brother would i still have to go? Even though he'd enjoy it alot more if he didn't have to worry about me "socialising" pfft with people twice as old as me- no thanks! Urgh it's anoying and i just don't want to go!! Help me ! And there's no option of just poping round because the venue is 20miles away.