Och, that sentence is a kind of indulgence of mine. The 4 ronin are all brothers! Jimmu was the legendary first emperor of Japan, and he had three brothers who featured in the tales of his adventures: Itsuse, Inahi, and Mikeiri. I wanted to reference the myth, but did so in a pretty clipped way, like I did with all the folklore here. I will mark that phrase down as another one to clarify for the after-judging revisions.
(And thank you, very much, for your kind review. <3)