Ancient Sages means more than one. Hence, it'd mean that Gaspar, Belthasar and Melchior had to have made it. But Dragonite sages could also count. And if for instance both were made out of Dreamstone, them being called brothers would be symbolic, even if they were not indeed crafted by the same person.
Magil clearly says it was created along with the Masamune, so it can only have been made by the same guy(s) who made the Masamune. I doubt it could be Dragonians, they don't even exist in RD. The Dragonians are from CC, a game which was made 4 years after CT (RD was made only 1 year after CT). Square didn't even intend RD to be a sequel to CT when they first started the game development, so it's hard to believe that Masato Kato already had the Dragonians and the whole Project Kid in his mind at that time.
Moreover, don't forget that the Gurus were called Sages in the Japanese version of CT. Belthasar in the American version of CC also states that he was once called the Sage of Reason. Thus I'm inclined to think that this RD quote is a direct reference to Melchior and his comrades.
Well CC was more of an official Sequel. It could very well be in RD that the Einlanzer was created by Melichior. But like has been rehashed over and over again, that isn't neccessarily what happened, except in another dimension.
The Einlanzer being a sister or brother sword makes sense in the Dragonian perpsective. Made as the Dragonian dimension version of the Masamune, they are very well brother/sister swords. I mean, who can say whether the Dragonians didn't have their own Melechior, Gaspar, and Belthasar. We can't really as they didn't have the Kingdom of Zeal and the problem of Lavos. They DID whoever have powers connected directly with the Earth.
If I understand correctly, Dragonians didn't have Dreamstone. And we really don't know what the Einlanzer is made from. It never says in CC. Dreamstone is the obvious choice, but why then does it have seemingly none of the Masamune's qualities? Or has too much good been imbued in it? If so then why wasn't it affected by Garai's vengeful ghost?
It points to perhaps being a relic from the Dragonian's dimension's past, or made after they crossed over (then having access to the Dreamstone.)