Very easy to do.

1) Check all bags through to DEN.
2) When you are done with Customs/Immigration at IAD, take your two bags and drop one bag off to the "Connecting Flights" Conveyer Belt. This bag will then meet you in DEN.
3) Take your 2nd bag and follow signs for EXIT / GROUND TRANSPORTATION (as if your final destination was IAD). Meet your brother in the unsecured area of airport. Hand him the bag. Re-enter airport as if you're flying a normal domestic flight -- TSA & all. Get to Denver and meet your bag.