I do not know why Dean’s brother was there. I have no insight. I have heard he was a civilian working for the US government, I have heard he was a tourist. Frankly, until the body was found, I did not even know Dean had a missing brother.
While I doubt he was a CIA agent, the CIA did take some people with backgrounds you would not expect. Read Robert Baer’s biography “See No Evil.” He was a CIA field agent who was hired in the mid-1970s. His mother was a relatively well off hippy (daddy supported her). She owned a left-wing book store in California. She also went on a long term trip through Eastern Europe.
When he applied for the CIA, he doubted his background would pass muster but it did. He noted that in class he had a real mixture of people. If I recall correctly, one of his classmates was a naval officer thrown out of the navy for opposing the Vietnam War.
The CIA wanted funky people. As he pout it, the CIA was willing to overlook the past and give people another chance.