Vibishan was a prince called satyavrat in his previous birth. He was called so as he always talked truth and walked in path of dharma. Vibhishana's sister Surpanaka was his gurus daughter in his previous birth and her name in previous birth is not known. She fell in love with Satyavrat. Satyavrat treated her like a sister and hence rejected her love. She felt insulted and became angry and vowed to revenge. She lodged a complaint in kings darbar saying that Satyavrat had a sexual union with her by promising her to marry and now refusing to marry her.
Nobody believed the innocence of the prince Satyavrat and thought that he lied. As because Satyavrat is not agreeing to marry gurus daughter, he was sentenced to death the day after.
Sitting in prison, satyavrat cried to heaven saying that there is no justice and dharma in the world. If there was any dharma in the world, he would not be sentenced to death and dharma could have proved victorious.
Then Lord Anand appeared before him and told him that because of his deeds in previous births, he will die now but in the next life he will be born in rakshasha race and his gurus daughter will take birth as his sister. This will happen so as he said to the king's darbar that he treated his guru's daughter as his sister.
That is why Satyavrat was born in rakshasha clan as Vibishan along with surpanaka as his sister. Vibhishana, like in this previous birth always walked in the path of dharma and started seeing Lord narayan's avatar in Sri Ram. Vibhishanas was ultimately blessed immortality by Lord Narayana. Surpanaka was punished by Lord Anand for her bad deeds.
Though a half-demon himself, Vibhishana was of a noble character and advised Ravana, who kidnapped and abducted Sita, to return her to her husband Rama orderly. When his brother did not listen to his advice, Vibhishana joined Rama's army. Later, and when Rama defeated Ravana, Vibhishana was crowned as the king of Lanka.
The Soul transmigrates from one birth to another with all its virtues of good and bad in nature that one has acquired during several lives in the past. It appears that Satyavrat truly treated his guru's daughter as his sister through thoughts and actions and hence she became his sister in his next life as Vibhishan. Satyavrat as Vibhishana also got the company of Lord Sriram as a result of his speaking truth always and following the right path of Dharma in the same way as Lord Srirama. Intensity of our thoughts and actions in the present life are likely to determine the location of our next life for improving and perfecting the same virtues.
Hence due to his nature accumulated not only in this birth, but also due to his Satwick character in his previous birth, he sided with Sri Ram, who is embodiment of Truth.
REBIRTH STORIES FROM EPIC VALMIKI RAMAYANA