Kristen Stewart's affair with Rupert Sanders had gone on for months, according to Sanders's brother-in-law.
The pair have both issued public apologies to their loved ones for their behaviour after they were photographed kissing.
Kristen, who was dating Twilight co-star Robert Pattinson, called her infidelity a "momentary indiscretion".
The brother of Sanders's wife Liberty Ross, Leopold, has alleged that the pair's fling began during filming of Snow White and the Huntsman last year.
He told The People: "It was from the last half of filming and all through post-production, clear into last week."
Ross added that his sister had made sacrifices for her husband's career.
He said: "She made some sacrifices for something she thought was worth it, now she knows, right? Five minutes from her home. Doubt it was worth it but it's life.
"It might actually make things better in the long run. She wasn't that happy for a while, but our family is close, she'll be all right."
Meanwhile, according to one of Ross's friends the actress and model " is prepared to give Rupert a second chance".
They told the Daily Telegraph: "She accepts that it was just one of those silly flirtations."
Pattinson has reportedly moved out of the LA home he shared with Kristen and is said to be feeling "heartbroken and humiliated."
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