إِذْ قَالَ لَهُمْ أَخُوهُمْ صَالِحٌ أَلَا تَتَّقُونَ
(Recall) when their brother Sālih said to them: “Will you not keep from disobedience to God in reverence for Him and seek refuge with His protection?” (Ash-Shu‘arā’ 26:142)
It is not only Prophet Sālih, upon him be peace, whom God mentions as “their brother,” that is, the brother of the people to whom he was sent. Although the people to whom each Prophet was sent were mostly unbelievers, God uses the same expression for Prophets Noah, Hud, Lot, and Shu’ayb, upon them be peace. What is meant by this expression is not that those Prophets shared the same belief, thought, worldview, and way of living with the people among whom they emerged or to whom they were sent. Neither was he from the same lineage as them. What is meant here is that every Prophet was chosen from among the people with whom he lived and that he spoke the same language as his people did. This usage may also be intended to stir up the feeling of compassion and appreciation in the people for the one who was chosen from among them as Prophet for their good, well-being, and happiness in both worlds. This usage also expresses a Prophet’s view of his people.
A Prophet may be regarded as the brother of his people as he is full of pity and compassion for them; he is also their fellow-countryman whose truthfulness, trustworthiness, chastity, and straightforwardness they knew very well. It is also because of this that a Prophet may be regarded and introduced as a brother of the people to whom he is sent.
The designations such as father, uncle, and elder brother could have been used for the Prophets. However, since these designations would call for respect and reverence, they would not have expressed the warm-ness of the word “brother.”
 See sūrahs ash-Shu‘arā’ 26:106; 26:124; 26:161; al-A‘rāf 7:85; Hud 11:84; al-Ankabut 29:36.