In the Aranya Kanda of the Ramayana, Rama, Lakshmana, and Sita are on the way to the ashram of the famous sage Agastya when they first stop by at the ashram of another sage:
While viewing beautiful forests, cloud-like mountains, lakes, and rivers that are flowing following the pathways, Rama happily journeyed on the pathway indicated by sage Suteekshna, then he gladly spoke this sentence to Lakshmana. "Definitely this alone appears to be the hermitage of Sage Agastya's brother, one with great soul and pious deeds. As how thousands of trees are bowing under the weight of flowers and fruit on the pathway, thereby I perceive it to be the hermitage of Agastya's bother." ...
Raghava spent one night there when that sage received him well and when they dined on tubers and fruits. Raghava spent that night there and when sun rose in solar orbit he took leave of the brother of Sage Agastya saying the following. "Oh, godly sage, we stayed in the night comfortably, I now make an obeisance and bid farewell to you, as I wish to proceed to see your mentor and elder brother Agastya. When the brother of Agastya said, "you may go," Rama the legatee of Raghu journeyed on the pathways as directed by Suteekshna[.]
My question is, who is this brother of Agastya that Rama visits in the forest? As I discuss in this answer, the sages Agastya and Vasishta are the sons of Mitra god of friendship and Varuna the ocean god. Here is how the Srimad Bhagavatam describes their birth:
Agastya and Vasiṣṭha Ṛṣis were the common sons of Varuṇa and Mitra, the tenth son of Aditi. Upon seeing Urvaśī, the celestial society girl, both Mitra and Varuṇa discharged semen, which they preserved in an earthen pot. The two sons Agastya and Vasiṣṭha later appeared from that pot, and they are therefore the common sons of Mitra and Varuṇa.
Clearly the sage Rama met wasn't Vasishta, since Vasishta lived in Ayodhya. So how could Agastya have another brother? Would he also be a son of Mitra and Varuna, and if so how was he born?
The Ramayana translator just says this:
The name of this brother of Agastya is Sudarshana. But none calls him by that name and he is just called agastya bhraata, Agastya's brother.
So are there any other scriptures that discuss Agastya's brother Sudarshana? And what text gives his name as Sudarshana?
EDIT: I found this chapter of the Setu Mahatmya of the Skanda Purana describes the death of Agastya's brother. In this account, Agastya's brother prays to Shiva for Moksha, and Shiva appears before him and tells him to bathe in the Amritavapi Tirtha, located in what is now the Ekantharama Temple near Rameshwaram. He does so, and he attains Moksha. But the Skanda Purana doesn't provide any biographical information about him, just that he is the "brother of the Pot-born Sage". So I'm still trying to find how Agastya, being born in a pot along with Vasishta, could have an additional brother.