His siblings have long retreated from the inner circle, but the president-elect’s eldest son has made a name for himself as the person who can best assess loyalty to the Trump political brand.

Robert F. Kennedy Jr. was a key figure in protests over California vaccine laws. He could set U.S. health policy in the Trump administration.

Trump, who fiercely prizes loyalty, has long relied on family members for political advice, but which relative has his ear is known to vary.

Donald Trump Jr. has emerged as the most influential Trump family member in the transition as the president-elect builds the most controversial cabinet in modern U.S. history, according to a half dozen sources with knowledge of his role,