Why was Ravan afraid of Vibhishan's daughter?

When Ravan kidnapped Sita and brought her to Lanka, Ravan's younger brother Vibhishan opposed it and called this the path to Ravana's end.

This angered Ravan, however, he didn't kill Vibhishan as the later was his younger brother. Moreover, Ravan was scared of Vibhishan's daughter Trijata.

According to the Purans, Trijata was Vibhishan and Sarma's daughter and a demoness. 

She was assigned the duty of guarding Sita who was kidnapped by the demon-king of Lanka Ravan.

According to myth, Trijata was powerful due to her devotion to Lord Vishnu and she could just burn Ravana into ashes with her power.

Legend has it that Trijata had predicted the Ram-Ravana war long time ago and had also forseen Ravan's killing.

Trijata's premonition angered Ravan, but he couldn't do anything due to his penance.

Trijata accompanies Sita on a survey of the battlefield of the war between Rama and Ravana. 

She reassures Sita of Rama's well-being when Sita sees her husband unconscious and presumes him dead.