Both have warned their brother that the 22-year-old actress may cheat again and he would be foolish to let her back into his life, reported Radar online.
"Robert's sisters were fuming with Kristen for hurting their brother in the way she did. 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," a source said.
"They have told him that there is no way he can take Kristen back after what she did to him. It's a case of once she has cheated, what is to stop her doing it again and Lizzy and Victoria are pretty unforgiving over Kristen's actions," it added.
"Instead, they believe Robert should embrace singledom, throw himself into work and not consider settling down with anyone until he is ready. They will kill him if he gets back with Kristen," the source said.