Why did Lord Vishnu have children with Mata Sita and even her mother Bhudevi (mother Earth)?

Why did Lord Vishnu have children with Mata Sita and even her mother Bhudevi (mother Earth)? Bhagvān Viṣṇu incarnated as Rāma and his consort Bhagvatī Lakṣmī incarnated as Sītā. The mother of all the worlds, Lakṣmī appeared as Sītā from the earth hence earth is considered the mother of the Sītā incarnation of Lakṣmī. This doesn't make Bhudevī the mother of Sītā in the way you are interpreting things. Rāma didn't have any other wife except Sītā so there's no question of children. On the other hand Bhūdevī is Viṣṇu's consort too, because she's an aspect of Lakṣmī. In his incarnation as Varāha, Bhagvān Viṣṇu becomes the father of his and Bhūdevī's offspring. Bhagvatī Lakṣmī divides herself into 3 aspects- Śrīdevī, Bhūdevī and Nīlādevī. This is mentioned in the scriptures. “The Goddess Śrī/Lakṣmī assumes a threefold form in conformity with the Lord’s will for the protection of the world. That she (Lakṣmī) is styled as Śrī and is known as such. The Goddess Bhū is the Earth comprising the seven islands and the seas; the container and the contents of the fourteen worlds such as bhū, etc.; and her essence is Praṇava. Nīlā is festooned with lightnings. To nourish all herbs and living beings, She assumes all forms." ~ Sītā Upaniṣad. “That Lakṣmī is the Earth only, well-known as goddess Nīlā. Being the support of the world, she has resorted to the form of the earth. She herself would be of the form of Nīlā due to her liquid form of the nature of water etc. She has obtained the form of Lakṣmī (Śrī). She is of the nature of wealth and speech.” ~ Padma Purāṇa. “As the consort of Viṣṇu, Śrī — the presiding deity of sattva-guṇa. As the presiding deity of tamo-guṇa, She is Durgā (Nīlā) or Kanyākā. As the goddess of earth, the presiding deity of rajo-guṇa, She is the consort of the Boar (Varāha). ~ Garuḍa Purāṇa. Thus Lakṣmī in the forms of Śrī, Bhū and Nīlā is the consort of Viṣṇu. As Sītā she's the consort of his incarnation Rāma and specifically in the form of Bhūdevī, she's the consort of his incarnation Varāha. Sītā's birth from the earth doesn't make Bhudevī her biological mother when the true self of Sītā, Lakṣmī herself is Bhūdevī. Thus Lakṣmī's incarnation appeared from one of her own aspects