Sarah's Key

In 1942, 10-year-old Sarah Starzynski (MÃ©lusine Mayance) hides her younger brother from French police by locking him in a secret closet and telling him to stay there until she returns. She takes the key with her when she and her parents are transported to the VÃ©lodrome d'Hiver, where they are held in inhuman conditions by the Paris Police and French Secret Service.
The deportees are transferred to the French-run Beaune-la-Rolande internment camp. The adults are deported to the Auschwitz concentration camp, leaving the children in the camp. When Sarah tries to escape with a friend, Rachel, a sympathetic Paris police guard spots them. When Sarah begs him to let them go so she can save her brother, he hesitates then lifts the barbed wire to let them out.
Sarah and Rachel fall asleep in a dog house at a farm where they are discovered by the farmers, Jules and Genevieve Dufaure. Despite knowing who they are and the associated danger, the Dufaures decide to help the girls. Rachel is dying, and when they call attention to themselves by calling in a doctor, a skeptical German officer asks them if they know anything about a second Jewish child. The officer begins a search for the second child, only to be interrupted when the French physician carries out the dead body of Rachel. Days later, the Dufaures take Sarah back to her family's apartment building in Paris. Sarah runs up to her apartment, knocking on the door furiously. A boy, 12 years old, answers. She rushes in to her old room and unlocks the cupboard. Horrified by what she finds, she starts screaming hysterically.
After the war, Sarah continues to live as a family member with the Dufaures and their two grandsons. When she turns 18, she moves to the United States, hoping to put everything that happened behind her. She stops corresponding with the Dufaures when she gets married and has a son, William. When her son is nine, Sarah â still despondent and blaming herself for her brother's death - drives her car into the path of an oncoming truck. It's explained to her son that her death was an accident.
In the present, the French husband of journalist Julia (Kristin Scott Thomas) inherits the apartment of his grandparents (his elderly father was the boy who opened the door to Sarah in August 1942). Having previously done an article on the Vel' d'Hiv Roundup, Julia finds her interest piqued when she learns that the apartment came into her husband's family at about the time of the Roundup and she begins to investigate what happened nearly 70 years earlier. Her father-in-law tells Julia what he knows so she will quit prying.
Julia begins an obsessive quest to find any trace of Sarah, eventually learning of her life in Brooklyn and finally locating William in Italy. She meets with him and asks him for information about his mother, but learns to her surprise that William does not know his mother's history or even that she was a Jew, believing only that she had been a French farm girl. Listening in amazement, William rejects the story and dismisses Julia. Later, everything is confirmed to William by his dying father, Richard, including Sarah's suicide. He gives William Sarah's journals and notes, telling him Sarah immediately had William baptized right after his birth, fearing that "being Jewish" was a threat to him and explaining that "...we're all a product of our history." The key to the cupboard is among the items handed to him by his father.
Julia, having given up hope of having another child after years of unsuccessful attempts to conceive, discovers she's pregnant. Her husband loves their life with their 12-year-old daughter, Zoe, as it is and does not want to have another child at this point in life. Julia ultimately decides against an abortion, has another daughter, divorces her husband and eventually moves with her new baby daughter to New York City.
Two years later, William, having contacted Julia, meets her for a late lunch in a restaurant favored by Sarah and gives her additional information about his mother that the Dafaures had. Julia is amazed and happy for him, and has brought her young daughter along to the meeting. William breaks into tears when Julia tells him her daughter's name is Sarah. Julia comforts him as they both look at little Sarah.

Question: Who is William daughter?
Answer: Sarah.; Sarah.