Pattinson, 26, was said to be reluctant to believe Stewart when she said she had only kissed Sanders who directed her in Snow White and the Huntsman.
"Kristen and Rupert both told their respective partners that they didn't sleep together. Kristen was absolutely steadfast that she only engaged in several make-out sessions with Rupert, but never slept with him," digitalspy.co.uk quoted a source as saying.
"Rob didn't believe her in the beginning, but as time wore on, he accepted her explanation. Kristen has given Rob passwords to her voicemail and e-mail account to prove she has nothing to hide, and will do anything to get his trust back," added the source.
Robert Pattinson and Kristen Stewart, who met on the set of the "Twilight", have reportedly reunited.
Earlier this week, Rupert Sanders's wife Liberty Ross was seen wearing her wedding ring for the first time since news of the affair broke, prompting speculation that they may also have reconciled.