A brother, sister and niece of Robert F. Kennedy Jr. on Wednesday chastised him for campaigning against vaccines, saying he has been disseminating “dangerous misinformation” that discourages immunization even as measles spreads throughout the United States.

They declared that they love him and praised his work to protect the environment. “However, on vaccines he is wrong,” they wrote in a column published in Politico under the headline “RFK Jr. Is Our Brother and Uncle. He’s Tragically Wrong About Vaccines.”

Mr. Kennedy’s sister, Kathleen Kennedy Townsend, a former lieutenant governor of Maryland; his brother, Joseph P. Kennedy II, a former Massachusetts congressman; and his niece, Maeve Kennedy McKean, executive director of Georgetown University’s Global Health Initiatives, criticized him for participating in social media campaigns that have stoked fear and mistrust of vaccines and for attacking public health institutions seeking to stem the spread of infectious diseases.

Mr. Kennedy, an attorney, author and environmental activist, said in a statement: “I am not anti vaccine. I want safe vaccines with robust safety testing.”