Robert F. Kennedy Jr.'s outspoken stance against vaccinations "has helped to spread dangerous misinformation," several of his own family members said in an op-ed published Wednesday.

"We love Bobby. He is one of the great champions of the environment," sister Kathleen Kennedy Townsend, brother Joseph P. Kennedy II and niece Maeve Kennedy McKean wrote in the Politico article. "However, on vaccines he is wrong."

His sister, a former lieutenant governor, his brother, a former congressman, and his niece, executive director of Georgetown University's Global Health Initiatives, said Kennedy is "complicit in sowing distrust of the science behind vaccines," potentially endangering lives.

They said "his and others' work against vaccines is having heartbreaking consequences. The challenge for public health officials right now is that many people are more afraid of the vaccines than the diseases, because they've been lucky enough to have never seen the diseases and their devastating impact. But that’s not luck; it’s the result of concerted vaccination efforts over many years."

The United States is in the midst of the largest measles outbreak in 25 years. Officials on Monday said 764 cases have been reported so far.

Robert Kennedy Jr. has been outspoken against some vaccinations for years, and last month announced his support of litigation aimed at combating New York City's efforts to force residents to get vaccinated because of a measles outbreak. "We are confident that no American court will allow government bureaucrats to force American citizens to take risky pharmaceutical products against their will,” Robert Kennedy Jr. said.

Robert F. Kennedy Jr. in the lobby of Trump Tower in Manhattan, on Jan. 10, 2017. Shannon Stapleton / Reuters file

He also testified in Washington state this year against a bill that would prevent families from claiming a personal or philosophical exemption to opt out out of getting their school-aged kids from getting the combined measles, mumps and rubella vaccine.

His relatives say that science has proven the vaccine is safe — and what's dangerous is what Robert Kennedy Jr.'s been doing.

"Those who delay or refuse vaccinations, or encourage others to do so, put themselves and others, especially children, at risk," they wrote.

It's also against family tradition. The trio noted in Politico that Kennedy's uncle, President John F. Kennedy, "signed the Vaccination Assistance Act in 1962 to, in the words of a CDC report, "achieve as quickly as possible the protection of the population, especially of all preschool children ... through intensive immunization activity."

"Bobby is an outlier in the Kennedy family," they wrote.