What a lovely idea!
I think the normal formal roles at weddings are beng relaxed more and more, especially as some coupled have one or both parents missing for various reasons.
You could say something about being the one best placed to say how wonderful your brother is - everyone will understand the unspoken message.
The great thing is, your speech needs to be heartfelt and short, in that order - it's for the best man to be the comedian on the day.
Your brother is a lucky man - enjoy your day.