When I first read the comic, it spoiled it when Optimus said to Megatron "Hello Brother." And Megatron said to Optimus "Hello Brother." In the movie when Megatron died Optimus said "you left me no chose brother." In Revenge of the Fallen, when the Fallen was fighting Optimus he said "Die like your Brothers." And Optimus said "They were your brothers too." Brother, What does that even mean to them?
Primus and Unicron semi-Gods originated from the same God/Semi-God source. They are like brothers, the yin and yang.
Brothers from another mother... board? I always assumed Megs and OP were "brothers" in the same way Obi-Wan and Vader were once "brothers" - as in they were close friends for much of their lives. The Primes may have been like this too (a brotherhood), though being 'born' in the same way at the same time would also make them brothers in a biological (or whatever) sense. The Fallen could've been speaking figuratively while Optimus was being literal.
Well here's what tfwiki sez: But I don't know where they get sibling from. You certainly can't definitively infer that just from Optimus calling him brother. The fallen was a Prime, so it's easy enough to say that the Primes are brothers. Perhaps Primes and High Lords are brothers as well?
In early scripts for the first movie, Optimus and Megatron were the sons of Sentinel Prime. The backstory is parred down in the final film to them having been friends turned enemies. When writing the second film, Optimus was the sole descendant of the Primes, hidden away from The Fallen. The brothers are presumably the ones The Fallen killed when Optimus was an infant. This too was left out of the final film. Clearly the writers understand people won't accept sexually reproducing robots despite the incredibly organic nature of the Baysformers, who moreso than any other Transformer come across as silicon-based lifeforms, people made of metal and energon instead of flesh and blood to be exact.
