K-Stew has described her infidelity to boyfriend Robert Pattinson as a "momentary indiscretion".
But The People newspaper reports Leopald Ross - the brother of Sanders' wife Liberty Ross - as claiming that the affair began last year and only ended last week.
"It was from the last half of filming and all through post-production, clear into last week," he said.
And speaking about his sister, he continued: "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."
Ross is reported to have forgiven her husband, but R-Pattz is claimed to still be angry and hurt.
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