the best way to dominate who's fitter need to combine a few test (as you suggested). ultimate fitness test must be triathlon it's include swimming, running and riding bicycles.

there are a few levels of difficulty "from kids of steel" till "iron man":

 you should choose one with your brother a level (I think that sprint or super sprint will be just fine and see who is the best). 

If you wish to read more and know the exact distances ream more in:http://en.wikipedia.org/wiki/Triathlon