I'm going to choose not to be a bridesmaid in my sister's wedding
Did something else conflict with the event?  Why would you choose something else over your sister's wedding?
I'd rather attend other social events than spend time standing at the bride's side all day.
It would be okay to miss such an important event if you truly had an unexpected emergency, but it is very rude of you to want to be with your friends rather than at your sister's side on the most important day of her life.  Family members, especially siblings, are expected to be there for one another's weddings.
You don't understand. I could be at some other party and it would be more fun.
Sometimes weddings can be a shore but you should still attend when it involves a family member. You should want to be there for an important moment of their lives. You would want people there to celebrate you when your time comes.