Sita Devi didn't name Vibhishana, Malyavan, Asumbanan or any other one while replying to Ravana in the Sundara Kanda.
The dialogue quoted in the between Ravana and Sita Devi occurs in the Sundara Kanda chapter 20, 21. Ravana speaks in the chapter 20 and Sita devi replies in the next chapter. The original verse for which you have posted translation is from Sundara Kanda sarga 21 shloka 9-10.
इह सन्तो न वा सन्ति सतो वा नानुवर्तसे || ५-२१-९
तथाहि विपरीता ते बुद्धिराचारवर्जिता |
Here are there not good people? Or are you not following those who are good? It must be like that because your mind, devoid of traditions is perverse.
She just meant good people during her conversation. She was not sure who were good or bad people in detail when she was talking this to Ravana. So, it is the translator who added these names while they were translating the original Sanskrit verse.
However, the people mentioned in the verse were of good qualities unlike the other rakshasas. They advised Ravana to make peace with Lord Rama and return Sita to Him. Here's what Vibhishana advised his elder brother Ravana regarding this issue.
Even before arrows resembling thunder bolts and with a speed equal to wind, dispatched by Rama, take away the heads of chiefs of demons, give back Sita to Rama. [6-14-4]
Even if you are protected by the sun god, or by Maruts (storm gods) nor even seated in the lap of Indra or god of Death nor if you have penetrated into the regions of heaven or under the earth you will not be spared by the arrows of Rama. [6-14-6]
Ravana turned a deaf ear to his words. From Yuddha Kanda Sarga 35,
Similarly, Malyavan, maternal grandfather of Ravana also advised the same.
I deem Rama of firm fortitude as Vishnu dwelling in human form. This Rama is not a mere human being, he by whom that most wonderful bridge was built across the sea. O, Ravana! Conclude peace with Rama, who is the king of men. Having come to know of his acts, let that which is good for the future be done after a mature understanding. [6-35-36]
Ravana also abuses him and turns his deaf ear. He gets ready for a war with Rama.
I couldn't find reference to Asumbanan in Ramayana. If there's a name variant, someone please let me know