He lived heavily loaded. If we look at functional alcoholics who are loaded (and overloaded) with people not really seeing their lack of functionality because they are functions, I think he was like a functional meth and morphine user. I think he was pretty convincing. Is he an animal like the rest? In my mind, absolutely. Is he calculating? Yep. Did he rape her? No. Did he kidnap her? Nope. Could he have saved her life? Yes. So he is an accessory and obstructed justice. He could have argued that if he tried to save her life that Zach would have killed him. According to him, Zach tried to have his brother killed.
Other thoughts:
When it comes to deviancy (of all forms), there is a plethora of cases where it is impossible to fathom how actions could have ever occurred to a human being to subject another human being to. For that reason, I believe that the incest, abuse, and crome toward others happened just as it was described.
I think Dylan won't testify because he knows that his brother was going to have him killed before and will probably have him killed in prison if he breathes a word to the authorities. I do think the other defendant was killed or forced to the point of suicide.