Board Thread:General Discussion/@comment-121.7.195.67-20161128145037/@comment-28150993-20161129172652

Expanding what Luckyeleven mentioned:

The Sanjou swords have different outfits and colours in general (the only similarity being that it's Japanese style with horse-riding hakama), unlike other "related" swords, e.g. Awataguchi and their double-breasted navy uniform, Rai and their black gakuran, etc. Same goes for their internal affairs outfits.

They seem to be familiar with each other in anthologies and the musical, but they call each other with honorifics (except Mikazuki who drops all honorifics with everyone) and never mention each other as siblings. In one story in the anthologies, Mikazuki even thought that the newly smithed Nakigitsune is Kogitsunemaru's brother because they're both kitsunes.

They also don't have any interaction in-game as siblings/family (we only have Iwatooshi and Imanotsurugi's recollection, but that's in their capacity as Yoshitsune and Benkei's blades, not as Sanjou). (Yet.) Hopefully such interactions will be implemented someday.

Even the two swords who are stated explicitly by the devs as having the same swordsmith (both Mikazuki and Kogitsunemaru are made by Sanjou Munechika, and the devs printed it out, while the other three Sanjou swords' smiths were never explicitly mentioned) do not explicitly acknowledge each other as siblings. Kogitsunemaru calls Mikazuki "Mikazuki-dono". Contrast that with the other sibling-swords like the Toushirous, Higekiri-Hizamaru, Kunihiro brothers, Samonji brothers, etc.

And well, if you don't mind spoiler, try checking Imanotsurugi's kiwame letters and its implications. It's very likely that Iwatooshi has the same predicament. As the player, it's kinda difficult to say that the Sanjou are brothers.

So I'd say, no, they're not really brothers and they don't explicitly consider each other brothers, but they have good relationship with each other.

Note: Sorry it got long. It's just that I've been very interested in this same question since I started playing last year. As a Sanjou fan, some of the revelations were quite... impactful.