I wanted to know if Mata Nui and Makuta Teridax ever had a personal relationship (before Makuta’s betrayal). The Matoran couldn’t have known about Makuta’s destiny to possess the Great Spirit Robot and work with Mata Nui to restore Spherus Magna, so why would their legends call Makuta his “brother?” Did Teridax personally interact with Mata Nui in his days as Miserix’s lietenant and develop such a relationship as to where it would be reasonable to call him his “brother” (unlike most Matoran Universe inhabitants who were largely removed from Mata Nui other than through their reverance)? Occasional dialogue by Makuta (for example, in Time Trap) suggests that Makuta knew Mata Nui on a level beyond seeing him as a being to overthrow for power.
This more a personal idea than an answer but I’ve always thought that the Matorans of Metru Nui saw Teridax like ancient Rome people viewed the Emperor: a being so powerfull and beyond them to be considered as a god.
Add to this the whole Barraki vs Makuta war in which Teridax undoubtedly came out as the hero who saved the island from the evil warlords and you may get a possible explanation as to why he was considered to be Mata Nui’s brother