Next, threatening to kill people is not a symptom of ADHD. Discipline may seem like an obvious way of controlling conduct, but it likely won't be effective unless all the adults involved agree on a strategy and apply it consistently.
This situation may be past the point that discipline alone will be a solution. You note that your stepson is violent toward his sisters. I realize you and your fiance don't have full-time custody, but when the kids are with you and their father, he has a clear responsibility to protect the girls - and you.
It seems an unreasonable expectation that you can play some immediately decisive role in a situation in which two other adults bear the primary responsibility. Is the boy receiving professional counseling or other treatment? Have you and your fiance met with a psychologist about this? I would strongly urge you to start with these steps.