The Company You Keep

A recently widowed single father, Jim Grant (Robert Redford), is a former Weather Underground militant wanted for a 1980 Michigan bank robbery and the murder of the bank's security guard. He has been hiding from the FBI for over thirty years, establishing an identity as a defense attorney near Albany, New York. When Sharon Solarz (Susan Sarandon), another former Weather Underground member, is arrested on October 3, 2011, an ambitious young reporter, Ben Shepard (Shia LaBeouf), smells an opportunity to make a name for himself with a national story. His prickly editor, Ray Fuller (Stanley Tucci), assigns him to follow up. Ben's ex-girlfriend, Diana (Anna Kendrick), is an FBI agent, and he presses her for information about the case. She tells him to look up a Billy Cusimano (Stephen Root). Billy, an old hippie with a history of drug arrests who runs an organic grocery, is an old friend of Sharon Solarz and a former client of Jim's. Billy is disappointed that Jim doesn't want to take Sharon's case, and he conveys this information to Ben when Ben questions him.
Ben pursues Jim and tries to question him, but Jim is evasive. Spooked by the federal investigation, Jim takes his 11-year-old daughter Isabel (Jackie Evancho) on "a little trip", driving north at first to throw off his pursuers. Ben learns that Mimi Lurie (Julie Christie), an accomplice in the Michigan bank robbery, was last seen in Canada. A private investigator helps Ben find out that Jim had no Social Security number prior to 1979 and finds a copy of Jim Grant's California death certificate. Ben concludes that Jim is really Nick Sloan, another former Weatherman, and writes an article breaking this news, creating a sensation and accelerating the FBI's interest.
Meanwhile, Jim and Isabel arrive in New York City. They go sight-seeing and check into a fancy hotel. The manhunt for Jim/Nick is now national news. While Isabel is sleeping, Jim leaves the room key hidden in the hotel lobby, where his brother, Daniel Sloan (Chris Cooper), retrieves it. FBI agents, under the direction of Diana's boss Cornelius (Terrence Howard), have been tracking Daniel and follow him to the hotel. They nearly catch Jim there, but Jim creates a diversion and escapes. Sharon Solarz refuses to provide any information to the FBI, but she agrees to talk to Ben. She is unrepentant about her radical activism in the Weather Underground; she reveals that Nick and Mimi had a love affair long ago. Ben also questions Daniel and writes a gratuitously unflattering piece about him, annoying Ben's boss.
Jim/Nick goes to Milwaukee to find Donal (Nick Nolte), his old best friend, who now owns a lumber yard. Donal discourages him from looking for Mimi, but tells him to contact former SDS member, history professor Jed Lewis (Richard Jenkins). Jed resents the Weather Underground's role in splintering the antiwar left. He initially refuses to help Jim, but when he finds out that Jim has a young daughter, he uses his connections with their old radical friends to track down Mimi. At Big Sur, California, Mimi imports marijuana into the U.S. aboard a sailboat as part of a big operation run by Mac McLeod (Sam Elliott), Mimi's boyfriend. Through Jed, Jim reaches Mac, who informs him that Mimi left to go "inland". Jim knows where she is going.
Ben begins to feel that Jim's actions make no sense for a guilty man. Defying his boss, he goes to Michigan to investigate the original crime. He meets with retired cop Henry Osborne (Brendan Gleeson), who was the first person to investigate the robbery. Osborne refuses to talk in front of his adopted daughter Rebecca (Brit Marling), and Ben realizes that he is hiding something important. Ben does some research, finding that Osborne had strong connections to Mimi's family before the bank robbery. He learns of the Linder-Lurie company property on the Michigan upper peninsula near Canada. Osborne acknowledges that if Mimi were to come forward with the information that Jim was not present at the robbery (although Jim's car was used during it), Jim would be cleared of all charges. Later, Ben flirts with Osborne's daughter Rebecca.
Jim and Mimi meet in a secluded cabin on the Linder-Lurie property; she is still passionate about the goals of the Weathermen and unapologetic about her actions 30 years earlier, but Jim argues that life has changed. Jim asks Mimi to turn herself in and confirm his alibi, for the sake of his daughter, Isabel; he doesnât want to leave Isabel behind and repeat the mistake that he and Mimi made 30 years earlier by giving up their own daughter. Mimi reveals that she saw their daughter recently in Ann Arbor, and that she is a beautiful young woman now. Meanwhile, Ben realizes that Jim is searching for Mimi, who can clear him of the charges, and that Rebecca is their daughter. He tells Rebecca to speak to her father.
The next morning, Mimi flees the cabin to sail to Canada just as Ben arrives at the Linder-Lurie property to find Jim. Ben tells Jim that he knows the truth about Rebecca and Osborne; Jim says that Ben must now decide whether to reveal it or keep the secret. Jim leaves the cabin so that the FBI will chase him instead of Mimi. Cornelius catches Jim, and soon afterwards, Rebecca learns of the circumstances of her adoption. Meanwhile, Mimi turns her boat around and returns to the U.S. to give herself up. The next day, Jim is freed from jail and reunites with Isabel. Ben decides not to expose Osborne's actions of 30 years before and to protect Rebecca's true identity.

Question: Who realizes Rebecca is the daughter of Mimi and Jim?
Answer: Ben.