I'm sure someone must have noticed this, i'm just posting these images to back up the theory.
Anyways I think that Sai's brother may be Suigetsu. One of the main connections is when Yamato says that "Sai, your brother was killed by none other than..." before being interrupted. The use of the words "other than" implies that its someone close, maybe not. When Yamato says that they are in Orochimaru's hideout, looking for Sasuke.
So basically I think that Sai's "brother" (not blood relation) is Suigetsu because Orochimaru captured or "killed" him, before cloning him or turning him into an experiment. Sai seems to deny any of this saying that his brother died of an illness.
The obvious gap here is that Suigetsu claims to have been taught by the 7 swordsmen of the mist, this holds up with his water based ability.
But I just thought that the resemblance was uncanny so I might as well post this. Another thing is that Sai's brother seems to have a sword attached to his back, or at least we see what looks like a hilt. THEY EVEN HAVE THE SAME HAIR-LENGTH!!! yea...
I'm sure i've missed loads of other things so please add/diss/rebuke/destroy.
Anyway here are some of the pics I am basing this on.
Sai's brother and Suigetsu:
Oh yea chapters 310 and 347 respectively
OK through some discovering of other people's theories on this on other forums, it kinda made me think about how Suigetsu=Sai, or how he got to where he is.
In the 2 images above, suigetsu and Sai's B don't look too different in age, the age Sai remembers him. Sai's picture book has a picture of himself with his brother, when they are young. He describes his brother as being "loud, impatient and unrefined", sounds like Suigetsu.
Unfortunately there are many clashes in this theory, asuming his brother is Suigetsu. Like when did he exactlly train with Zabuza and his origins.
There's a picture of him when he's small, when he's with Sai, so it obviously wasn't previous to that. He also joined with Orochimaru after Zabuza died at least 1 year previous to Sasuke's defection.
He was in Roots with Sai, Sai even paints him with a Konoha headband. Also, Danzo submitted Sai and presumably everyone in Roots to the same exam taken in the Hidden Mist, CONNECTION! woo
Anyways I think that Danzo's techniques, subordinates and ideologies (anti-Konoha) all link in to the Hidden Mist. Maybe he had Suigetsu trained abroad and then faked his death.
Oh godammit this theory goes so deep! I mean, I'm having to research the connection between 3rd Hokage, Orochimaru and Danzo! gahh
This edit has taken me over 40 minutes already! >_<
OK, research done.
When Sai gives Orochimaru the letter from Danzo, Orochimaru says that he once knew Danzou. Danzou was opposed to The 3rd whilst Orochimaru was still in Konoha, that means that they could have been allied. With Danzou's connections outside of Konoha, Orochimaru would be able to have allies on the outside for when he defected.
OK MY FINAL THEORY!!!
Suigetsu was a ninja from Konoha who got trained by roots under Danzou's command. Danzou at some point found out about Orochimaru's experiments and plans and pretended to ally himself with Orochimaru against Konoha before Orochimaru left. Danzou would send potential shinobi to train in the Hidden Mist by faking their deaths with orders to kill Orochimaru. When Yamato says that his "brother was killed by none other than..." could mean that Konoha somehow found out that Danzou was killing his subordinates, but somehow when his cover got blown, he revealed the plan to defeat Orochimaru from the inside as a way to help Konoha, much like Sai's secret mission. Konoha did not agree with these methods and disbanded Roots. Suigetsu was one of these potentials and he spent his time training with Zabuza. He wasn't sent before Orochimaru defected because that was over 12/13 years ago. So Suigetsu spent his time training in the Hidden mist and then somehow got captured/joined with Orochimaru who then experimented on him and could be the reason behind his ability. Suigetsu expressed wishes to kill Orochimaru to Sasuke, saying that he had no chances.
And he grew the teeth whilst he was in the Hidden mist, something in the water...
AAHHHH that took so long
Please dont find a gap in the theory, it took me so long -_-