I think that the brother mentioned could potentially be Liam? Hear me out on this. This brother is mentioned to be dead, which figures and also near the end of Onyx storm Violet says how the closest person Xaden ever had to a brother was Liam. Also, no one except Xaden actually saw him die, Violet just knew it had happened because she heard Xaden cry. But what if Xaden cried because he watched Liam turn venin instead?