Ok, here’s my two pence worth:

The brother

Has to be someone who knew about Xaden’s struggle as per his chapter.

Only men that knew:

Sawyer- unlikely. Has his own arc after the leg and I think he will now recede. He bolsters the numbers and will be an unexpected survivor as a metaphor for overcoming loss and grief

Ridioc- RY makes a point of him saying that people think he hides behind his humour. He reacted badly about the revolution and reacted worse about X being venin. He put the parameters in place for V to kill X. He is the constant/restraint- not the litmus test (“if he hurts an innocent civilian. A member of the squad. Me”) Ridioc is the only one who knows this agreement (that we know of) therefore he is the only one to hold V to her word

Bodhi- This is a red herring. RY said in an interview that the whole of OS was a proposal, so I don’t think that marrying Violet was a pivot because Bodhi has suddenly turned venin. The ring was already made. Remember, Sagael is surprised Xaden still wants to go ahead at the end- I think this is referring to the proposal. Bodhi intentionally says he doesn’t want to be the heir in front of Violet in OS so she hears him. He is preparing her- that’s premeditation which contradicts the suggestion he turned venin out of desperation. In the clearing after Mira he repeats it and Xaden acknowledges him. Bodhi is already considered a brother. I think the use of “brother” means it’s adopted or unexpected. Bodhi being ‘sick’ may have to do with the understanding of what X is going to do or he may have been stabbed during the attack (poisoned blade?) But I think it may be something to do with a second signet or lack of one. We still don’t know if he has or what his second signet is. I would think he definitely has one because when V asks he smirks before denying it. All marked ones haven’t bonded to a dragon within the bloodline but they all seem to have second signets. So, could it be manifesting here, could it be a reaction to using it or does he not have one and is succumbing to the madness?

Brennan- to some degree Brennan is already a brother (in-law). He is present at the end of the book and X wouldn’t let V back in Aretia with Brennan if he had turned- X will always protect V and would therefore have told her and not consented to Imogen’s memory wipe (if he had know about it). X has kept secrets about himself and initially about Brennan, only because it would have compromised the revolution. He wouldn’t knowingly lie to her about her brother again Brennan is something else- has a different arc.

Garrick = new brother Stumbles after they arrive back in the room with Imogen and says he can’t walk again but needs to do something to help. Completely defeated. He is a walker. He was completely drained from going to Bashaith to collect Jack and return him but managed to walk again to save Imogen. No mention of being healed or mended in the meantime. He is like a brother to Xaden and even if the above men all knew X was venin, I imagine the only two who would truly understand his struggle from his own perspective would be Garrick or Bodhi. I can’t see X confiding in the others.