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 :
māṁ viddhi mūlaprakr̥tiṁ sargasthityantakāriṇīm .
tasya sannidhimātrēṇa sr̥jāmīdamatandritā ..
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.