Rupert's brother-in-law has told UK paper The People that the duo were hooking up for months on the set of Snow White.
"It was from the last half of filming and all through post-production, clear into last week," Leopold Ross told the paper.
The film shoot ran from mid-September to mid-December 2011 and was followed by months of post-production work, which means they could have been having "momentary indiscretions" for a good six months.
Leopold is the younger brother of Rupert's 33-year-old wife Liberty Ross.
He also told the paper that Liberty "wasn't that happy for a while" in her marriage to Rupert.
But another source told the UK's Daily Telegraph that Liberty has forgiven her cheating husband.
"[She] accepts that it was just one of those silly flirtations," said the source.
"[She] is prepared to give Rupert a second chance."
It seems Robert Pattinson isn't so prepared to forgive and forget.
Moving trucks were spotted departing from the home he shared with K-Stew on Saturday morning, suggesting the "devastated" Twilight star has officially called it quits on their relationship.
Kristen has reportedly also moved out of the couple's $6 million LA pad.
A source told Radar Online the 26-year-old wants to "find out exactly what happened between [Rupert] and Kristen" in a "man-to-man" chat with the 41-year-old director.
And in another twist, Radar reports that K-Stew's Snow White co-star Charlize Theron is also peeved about the tryst.
The 36-year-old reportedly "felt like an older sister to Kristen" and even asked the young actress to be godmother to her newly adopted baby Jackson.
"Charlize had no idea what was going on between Rupert and Kristen and now feels completely betrayed by the whole incident," a source reported.
Kristen Stewart — breaking hearts left, right and centre.
