There are various versions of the Ramayana.
There are some variations in the narrations among them.
Most notable among them, I thought was the one, which states that it was not the real Sita but Maya Sita.
But, Mandodari, who is considered to be a Chaste woman and kept on par with Arundhathi, Vasishta’s wife as a model of Faithful wives got married to Vibhishana, after Ravanas’ death, at the instance of Lord Rama.
This is from the Rama charita Manas by Tulsidas.
Mandodari was considered as one among the Pachkanya who were known for their chastity.
“After the death of Ravana, Rama advises Vibhishana to take Mandodari as his wife, even though he already has a wife. A theory suggests that Ravana’s race may have had matrilineal families and thus, to restore order in the kingdom after Ravana’s death, it was necessary for Vibhishana to marry the reigning queen to get the right to rule. Another theory suggests it may be a non-Aryan custom to marry the reigning queen. The marriage between Mandodari and Vibhishana is purely an “act of statesmanship”, rather than a marriage based on their “mutual sexual interference”.Mandodari may have agreed to marry Vibhishana, her younger brother-in-law, as this would lead the kingdom to prosperity and stability as allies of Rama’s Ayodhya, and she would continue to have a say in governance. Another reason for the marriage is as an alternative to suicide for the widowed Mandodari, which is averted by Rama.
Adbuda Ramayana states that Mandodari was the Mother of Sita.
The Adbhuta Ramayana narrates: Ravana used to store the blood of sages he killed in a large pot. The sage Gritsamada was practicing penance to acquire the goddess Lakshmi as his daughter. He stored milk from Darbha grass and purified it with mantras in a pot so that Lakshmi would inhabit it. Ravana poured the milk from this pot into his blood pot. Mandodari is frustrated seeing the evil deeds of Ravana, so she decides to commit suicide by drinking the contents of the blood-pot, which is described to be more poisonous than poison. Instead of dying, Mandodari gets pregnant with the incarnation of Lakshmi due to the power of Gritsamada’s milk. Mandodari buries the foetus inKurukshetra, where it is discovered by Janaka, who named her Sita.
Valmiki’s Ramayana does not say anything to this effect.
Mandodari married Vibhishana Ramaharitamanas Link.