It was revealed on Thursday that 41-year-old Sanders had an affair with the star of the film, Pattinson's girlfriend Kristen Stewart. Both he and Stewart subsequently released statements apologising for their behaviour.
"Rob wants to have a man-to-man chat with Rupert to find out exactly what happened between him and Kristen," a source told RadarOnline.
"Kristen has already betrayed his trust by cheating, so he can't be sure that she will tell him exactly what went on between her and Rupert.
"Rupert has a wife and a family to save so Rob feels like he has nothing left to lose and would be honest with him."
Stewart and Pattinson are thought to have been dating since 2009 after meeting on the set of the Twilight films. Following last week's revelation of the affair, which Stewart described as a "momentary indiscretion", Pattinson is said to have moved out of the LA house they shared.
"Rob really was blindsided by this and he's going through the typical stages of being cheated on," the source continued.
"He was hurt and heartbroken, then angry and p*ssed off, and now he just wants answers. He feels like he needs to know exactly what happened, how far it went and how many times, so he can figure out if this is something he can move past.
"Rob just doesn't know who or what to believe right now and he'll do whatever it takes to find out the truth.
"He's not ready to talk with Kristen, it's too painful, so he's going to Rupert."
The final instalment of the Twilight film series, Breaking Dawn Part 2, is due for release in November. Both Stewart and Pattinson are said to be contractually obliged to attend the premieres and pose for photos side by side on the red carpet.
Watch a teaser trailer for Breaking Dawn Part 2 below: