Farrow told Vanity Fair magazine that she and Sinatra "never really split up." When she was asked directly if Sinatra was Ronan's father, she responded "possibly."
Ronan was born in 1987 during Farrow's long relationship with Woody Allen, who was assumed to be his biological father.
Farrow also has three biological children from her marriage to conductor Andre Previn and has adopted 11 children, including two who have died.
Nancy Sinatra, who is almost a half-century older than her possible half brother, told Vanity Fair she thinks of Ronan as a family member.
"He is a big part of us, and we are blessed to have him in our lives," she said.
Sinatra and Farrow divorced in 1968, two years after their marriage. Farrow was married to Previn from 1970 to 1979 and involved with Allen from 1980 to 1992.
The relationship with Allen ended when Farrow learned he was in a sexual relationship with Soon-Yi Previn, her adopted daughter and now his wife. She accused him of molesting another daughter, Dylan Farrow, who was then 7, a charge Allen denies.