Personally, I think the biggest downside would be that you're going to miss out on the wedding and reception. You're going to end being so focused on taking pictures that you won't really get to participate. Is that something you're okay with?
If you okay with that and they are truly okay with you shooting the pictures rather than a pro, then do it up! Even if you only do a mediocre job at least you're going to have more invested than a typical friend being that he's your brother so you'll probably do a nicer job. cjric does brings up some good points to think about though.