You probably don't need agreements in place with your brother before you start - but you ought to think about doing some paperwork with the company whose webpage he's designing, because you don't want to end up with them pissed off at you because their web pages become unavailable. You want it to be clear that you're doing them a favor and can stop whenever you want - what if they get a ton of traffic and send your colo bill sky high, or get the colo people mad at you? They also shouldn't get the impression that the way things normally work is that they pay someone to put together a website and get free hosting eternally - it's great if you and your brother want to do that right now, but that's a pretty difficult expectation to support over the long run.
it's great if you and your brother want to do that right now, but that's a pretty difficult expectation to support over the long run.
I would also have an agreement with your brother concerning what he can expect from you and what he can't. This helps to ensure you remain on good terms with your family and helps to avoid having something as silly as the hosting of an account bring contention between you and him.
Families have broken up over lessor things and it's important to do everything you can to keep your relationships in tact over and above all else.