How to avoid going to my step-brother's wedding?

So, my step father's son is getting married this friday and I'm really NOT interested in attending, because we're not close at all, he doesn't even congratulate me on birthdays and stuff like that, I talk to him like twice a year and I'm completely sure that I won't enjoy being there. My mom and my step-father told me it was "obligatory", I'm 18 years old for god's sake, I don't know what excuse I could use...