Even though the brother of Jared plays a crucial role in this narrative, the focus is on Jared’s lineage because Ether was a direct descendant of Jared. There is no mention of the brother of Jared’s name in the Book of Mormon text. Joseph Smith later revealed that it was Mahonri Moriancumer.
Ether knew, as a prophet himself, how important the faith of the brother of Jared was. Without that faith, the Jaredites would never have arrived at their new land or known of the promises of the Lord.
One may ask why Jared, who clearly had faith too, did not seek the guidance himself. Jared sent his brother repeatedly to consult with the Lord on behalf of himself and their families. It appears that there may have been a priesthood function involved. In the ancient world, families would dedicate some of their children to serve in the temple. We also know that temple building and seeking heaven was a big part of this culture, though the people of Babel went about it in the wrong way.
Temple priests served an important role in Mesopotamian society. In the ancient Mesopotamian world, they even sold temple franchises. The temples were an economy that had to be managed. Priests would have stewardships over crops, land, and animal. They had to be able to hold the proper festivals. They needed to support themselves so that they could attend to the functions of the temple and the religion. It was a great honor for a family to dedicate a child or two to this service. The priests probably did not make a lot of money, but they certainly garnered great prestige.
Perhaps Jared and his family had a connection of that nature through the brother of Jared. He may have been involved in the religious worship and he had faith necessary to be able to get the answers and blessing from the Lord that he needed.
The brother of Jared received some truly great revelations about the wonderful mysteries of things from the beginning to the end, and they were clearly new to him. Without his brother’s faith and savoir-faire, Jared would not have had the critical information needed to conduct the expedition.