And his sisters Lizzy and Victoria Pattinson are particularly incensed about the high-profile breakup which humiliated their famous sibling.
"Robert's sisters were fuming with Kristen for hurting their brother in the way she did," a source told RadarOnline.
"Lizzy and Victoria feel terrible for heartbroken Robert, especially because of the embarrassment that was caused by the manner in which the affair was exposed.
"It's a case of once she's cheated, what's to stop her doing it again and Lizzy and Victoria are pretty unforgiving over Kristen's actions."
The insider also added that the star's sisters are adamant that he should never reconcile with his Twilight co-star.
"They've told him there's no way he can take Kristen back after what she did to him," the source said adding that they told their brother, "Don't even think about getting back together with Kristen Stewart."
Lizzy and Victoria also believe "Robert should embrace singledom, throw himself into work and not consider settling down with anyone until he's ready."
"They will kill him if he gets back with Kristen," the pal added.
Robert is already showing signs of moving on and has placed the $6.3 million L.A. mansion he once shared with his ex on the market and is apparently "thinking long and hard about returning to London."