Because they were described as worshippers of the thicket, it would not be fitting to mention brotherhood here; but when they are referred to by their tribal lineage, Shuʿayb naturally became known as their brother. This distinction is a rare, subtle, and precious insight.
As for the argument that the reference to “the Day of the Canopy” proves they were a separate nation, then by the same logic the mention of the earthquake and the blast would prove two other nations—and no one with any real understanding would say such a thing. (1)
The ḥadīth cited by the ḥāfiẓ Ibn ʿAsākir in the biography of the Prophet Shuʿayb, peace be upon him, (2) comes through the chain of Muḥammad b. ʿUthmān b. Abī Shaybah—from his father—from Muʿāwiyah b. Hishām—from Hishām b. Saʿd—from Shaqīq b. Abī Hilāl—from Rabīʿah b. Sayf—from ʿAbd Allāh b. ʿAmr—in marfūʿ form:
“The people of Midian and the people of the thicket were two nations; Allah sent to both of them the Prophet Shuʿayb, peace be upon him.”
This report is odd, and several of its transmitters have been criticised. It is more likely that the statement is the saying of ʿAbd Allāh b. ʿAmr himself, taken from the two saddle-bags of Israʾīli traditions he obtained on the day of Yarmūk. Allah knows best.
Moreover, Allah condemns the people of the thicket for the same offence—cheating in weights and measures—that He attributes to the people of Midian. This shows that they were one nation, destroyed by a variety of punishments. In every passage He mentions the form of retribution that best suits the context.
With regard to His words, “So the punishment of the Day of the Canopy seized them—it was indeed the punishment of a mighty day” (al-Shuʿarāʾ 26:189), it is related that an intense heat struck them, and Allah withheld every breeze from them for seven days. Neither water, nor shade, nor retreating into underground chambers gave them any relief. They fled their dwellings to the open desert, where a cloud spread its shade over them. They gathered beneath it, and once they were all assembled Allah made the cloud rain down sparks and flaming missiles upon them. The earth quaked beneath them, a deafening cry descended from the sky; their souls were taken and their bodies destroyed:
“So the quake seized them, and by morning they were lying lifeless in their homes, fallen upon their knees. Those who rejected Shuʿayb were as though they had never lived there; those who rejected Shuʿayb—they were the losers.” (al-Aʿrāf 7:91–92)
Allah saved Shuʿayb and those who believed with him, as the Most Truthful says: “When Our command came, We delivered Shuʿayb and those who believed with him by a mercy from Us, while the Cry seized those who had wronged, and by morning they were lying lifeless in their homes, as though they had never prospered there. Away with Midian, just as Thamūd was cast away.” (Hūd 11:94–95)
He, exalted is He, also says: “The arrogant leaders among his people who disbelieved said, ‘If you follow Shuʿayb you will surely be the losers.’ So the quake seized them, and by morning they were lying lifeless in their homes, fallen upon their knees. Those who rejected Shuʿayb were as though they had never lived there; those who rejected Shuʿayb—they were the losers.” (al-Aʿrāf 7:90–92) This was the perfect rebuttal to their own taunt, “If you follow Shuʿayb you will surely be the losers.”
Thereafter the Almighty recounts that their prophet announced their ruin to them, rebuking, admonishing, and censuring them, saying: “O my people, I have surely conveyed to you the messages of my Lord and given you sincere counsel…"