I'm not going to hold up my brother's courthouse wedding
Why do you think you will not hold up the wedding?
I'm the witness for the groom, and I'm making sure I'm there on time.
That's very good of you. I'm sure your brother will be happy to hear that. Helping your brother on his wedding day is the right thing to do.
I am really more worried about myself than him.
It's understandable to be nervous when your brother is getting married. I'm sure everything will be fine, though, so I wouldn't worry too much.