The 33-year-old journalist took to Instagram on Sunday to share a photo of himself posing with Dylan, 35.
"Proud of my sister," Ronan captioned the shot.
He shared the post on the day that HBO premiered the first episode in its four-part docuseries "Allen v. Farrow." The project looks at the contentious 1992 custody battle between Woody Allen and Mia Farrow over children Ronan, Dylan and Moses Farrow.
Dylan has previously claimed that Allen sexually abused her when she was 7 years old. In the "Allen v. Farrow" trailer, she said she hopes the series will address "so much misinformation" regarding this period of her life.
It has been nearly two decades since the "Midnight in Paris" director was first accused of molesting his adopted daughter. Allen has never been arrested or prosecuted, and he has continually denied the accusations. Back when the accusations initially went public in the 1990s, he said in a statement, "This is an unconscionable and gruesomely damaging manipulation of innocent children for vindictive and self-serving motives."
Allen has maintained that Mia made the accusations to retaliate after learning the Oscar-winning filmmaker and her adopted daughter Soon-Yi Previn were in a romantic relationship. Mia denied this claim.
Ronan, who appears in the HBO series, has previously offered public support for Dylan. In 2016, two years after she wrote an open letter for the New York Times detailing her allegation, Ronan penned his own essay for The Hollywood Reporter in which he stated, "I believe my sister."
After the first episode aired on Sunday, a spokesperson for Allen and wife Soon-Yi released a statement to media outlets criticizing "Allen v. Farrow."
"These documentarians had no interest in the truth," the statement read in part. "Instead, they spent years surreptitiously collaborating with the Farrows and their enablers to put together a hatchet job riddled with falsehoods."