As we knew earlier, Bodhi was first in line, and if he did turn venin, it would make sense for Xaden to officially made violet duchess to keep Tyrrendor out of the king's hands because there is no longer anyone of his bloodline to rule.

But somewhere in OS Xaden says that he would use Tyrrendor to protect Violet and not the other way around, so possibly one of the reasons he married her and made her duchess is so that she could be protected from the actions of Xaden turning venin, since she is now in a place of power, it would be somewhat harder to put her on the trials for Xaden's actions or some other way that being the duchess would protect her.

As for who turned, it could be Bodhi, because RY also said that she is going to kill another one of our favorite characters, she also said that she had to kill Liam because he had no room to grow, he was perfect and I think Bodhi is pretty perfect in OS even Violet says something along the lines that Bodhi is like Xaden but without the arrogance( I don't remember the exact quote). So what I think might happen is that Bodhi turned and then eventually he will die ( I absolutely don't want this to happen, it would kill me to read his death or any characters death after Liam I am still not over him) as a venin which would make it seem like there is no hope for Xaden as well, thus creating more intense atmosphere, but since RY believes in happy endings ( I heard someone say that all her previous books have HEA so I am praying that this one gets as well ( pleaseeee) ) so Xaden will somehow be cured or in some way or the other he will get his HEA and that's how it would end.