Hitler had several living family members, most notable perhaps being his nephew William Patrick Hitler, the son of Hitler's older half-brother Alois, Jr. Born to a British mother, William grew up in the United Kingdom. When his Uncle first rose to power, hoping to ride the coat-tails, William returned to Germany to look for a position - by some accounts to use 'family dirt' to essentially blackmail Hitler into giving him one. Adolf did help him find essentially minor employment initially, but he was far from an enthusiastic Nazi, quickly resulting in his uncle ceasing further help, and turning antagonistic, William even being jailed briefly. He finally left Germany in early 1939, moving to the United States, where he began to write and speak against his uncle.
In mid-1941 as a British citizen, he attempted initially to join the Canadian military but was rejected, apparently telling him his anti-Nazi lectures, which attracted large crowds and fundraising, were more worthwhile of his efforts, although other reports indicate that there were doubts as to his sincerity, as at least some believed he was simply profiting off his name and the gesture was more about publicity. Shortly after American entry, he then made a personal appeal to FDR requesting he be allowed to join the US military. He was initially rejected, and it wasn't until two years later, however, that he was finally allowed to enlist, being inducted into the US Navy. After the war he changed his name to Stuart-Houston, got married, had some kids, and settled on Long Island. Although his children never had children themselves, the rumors that they made a pact to let the 'Hitler' bloodline die out is unsubstantiated at best.
So in short, there are essentially two views of William Patrick Hitler. By his own reckoning, he was hopeful of the promises made by his Uncle of restoring German pride, but almost immediately soured on it when he saw the truth of the regime. Persecuted, he eventually fled Germany to lecture on the threat Nazism posed to the World. Alternatively, he was an opportunist who wished to profit off his Uncle, first in Germany, but when he was unable to go far on the family name there, instead in America where it was an asset of sorts. In either case, we can say for certain that early on he was willing to benefit off his Uncle, and whatever the reason for falling out, he was certainly a notable public voice against him by the end of the 1930s.
Almost all from his FBI files, which are publically available