"I love and support my sister and I think her words speak for themselves," Tweeted Ronan, 26, on Sunday, just prior to posting a series of Tweets on the Super Bowl.
In her letter, Dylan, 28, who had previously made allegations of abuse in a Vanity Fair interview in October, details the ways in which the claimed assault affected her.
"That he got away with what he did to me haunted me as I grew up," she says in The New York Times. "I was terrified of being touched by men. I developed an eating disorder. I began cutting myself."
Allen has denied the abuse and plans to respond publicly to Dylan's claims.
"Mr. Allen has read the article and found it untrue and disgraceful," his rep told PEOPLE.
Towards the end of her letter, Dylan calls out actors from Allen's movies, including Cate Blanchett, who is nominated for a Best Actress Oscar for her role in Blue Jasmine.
"What if it had been your child, Cate Blanchett?" She writes.
Asked at the Santa Barbara International Film Festival Saturday for a reaction to the letter, Blanchett said, "It's obviously been a long and painful situation for the family and I hope they find some sort of resolution and peace."
I love and support my sister and I think her words speak for themselves.— Ronan Farrow (@RonanFarrow) February 2, 2014