I thought it was more of an older brother/younger brother, but you are right, they did reverse the ages in the film. In the book, Sam is younger than Frodo (who is about 50 in hobbit-years, which would be ~ thirtysomething in human years)
I think you're probably closer on that ... my boys are all still at the "fight like wolf puppies" stage, so I wasn't thinking of brothers :-).