In the cartoon by Mike Young Productions they never said it on-screen that Randor and Keldor were brothers. They gave small mentions here and there and story-editor and writer Dean Stefan has mentioned in an interview at Masters of the Universe Chronicles-podcast, that one idea they wanted to execute; was a flashback with young Randor and Keldor.
It would have shown Keldor being the "bad seed" of the family with possible attempt at poisoning their father, Miro. Now that flashback sounds like a great concept.
Also the people at MVCreations were planning to do a prequel mini-series comic that would have highlighted young Captain Randor VS Keldor. The story would have been written by Marv Wolfman and artist Andy Tong was attached to do the pencils.
The prequel would have been four issues, published in early 2004. Marv Wolfman has recollected at a Baltimore Convention in 2005 that he had lot of fun writing it and proud because it was a character-driven story. Unfortunately he couldn't remember a lot about the story as he had misplaced the scripts.
Apparently 3 out of 4 issues had been written before the project was ended. First issue would have started with a six-month year old Prince Adam, as Randor was off to "do his thing", which possibly means fight in the war against Keldor.
Artist Andie Tong has stated that:
"Sadly [I] did not receive the script, nor know the main jist of the story. Was just told what to design. Started designing but it was canned shortly after. So didn't even really get into designing the characters proper.
Never got to do any of the interior pages."Hopefully one day the scripts by Marv Wolfman could be located so fans would get a glimpse of the prequel-story...