Divine Birth of Seetha
Sita was born in the worthiest family of Janaka. Every member of Janaka’s lineage was of righteous conduct and diligently abided by all good traditions and customs. While ploughing the field, King Janaka found a pitcher, within which was a baby girl. As such, Seeta was ayonija (not born from a mother’s womb in the normal course).
Mahalakshmi Manifested herself as Sita by her own wish.
Whenever Lord Vishnu takes Incarnation, She also appeared to assist him in his divine play.
Here is verse from Adhytma Ramayan,
where Sita says Herself says that she is Adi Prakriti :
Know me to be the primeval Prakriti, created, preserver and destroyer . By mere proximity to him (Lord Rama) , I watchfully create all this.
So, Without a Second, Sita is Herself Mahalakshmi. She is also known as Mahamaya, She is original Source of Maya (illusion). So how can She forget herself ?
In Valmiki Ramayana Lord Bramha himself says :sītā lakṣmīrbhavān viṣṇurdēvaḥ kr̥ṣṇaḥ prajāpatiḥ||
Sita is no other than Goddess Lakshmi, the divine consort of Lord Vishnu.
Did Others Know about it ? Not Fully - It was a Divine Secret
While King Janaka showered all the love and affection and knew that Sita is Divine. But that is about it.antarikṣē ca vāguktā apratimā’mānuṣī kila .
(Valmiki Ramayana )
(Thereupon, a voice from the skies said: "As you said, O, King! Let it be so. This divine (amānuṣī) child without a match (apratimā) is a legitimate daughter to you, rightly.
In Venkatachala Mahatmya of Skanda Purana, Vishnu describes the events happened in the Treta Yuga.
While Ravana took Vedavati to Lanka thinking she was the real Sita, while Agni took Sita to his house and asked his wife Swahadevi to look after her.After the destruction of Ravana, Vedavati entered the fire when rejected by Rama.Then, Agni, offered the real Sita to Rama.
Rama then questioned her as to who the other lady by her side was. Sita informed Rama that the lady was Vedavati who endured Ravana's torture for ten months in Lanka for her sake.
He says that it was Vedavati not Janaki who was abducted
24-26. Afterwards when Ravana was killed, she entered fire once again. Agni handed over Lakshmi, my Janakl, who had been kept in her protective custody by Svaha and told me about the attendant accompanying Sita:
“O Lord, this is Vedavati who did what was pleasing to Sita. For the sake of Sita she remained in the abode of the Rakshasa as one imprisoned by him. Hence, delight her with a good boon”.
You can read about this on the TTD official site at https://www.tirumala.org/srk12.aspx