The 26-year-old actor, who recently moved out of the house he shared with Stewart after she publicly confessed of cheating on him, wants to find out what exactly happened between him and Stewart, reported Radar Online.
"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," a source said.
Sanders, 41, is married to model and actress Liberty Ross and they have two children together.
Pattinson and Stewart will be reunited at the premiere of the final instalment of the "Twilight Saga, Breaking Dawn Part 2".