Well, I honestly don't know what you can
do at this point. Obviously, unless there was some egregious piece of information you left out of your question, this is light years away from being your fault. Your stepmum must need somebody else to blame because it is too difficult for her to point the finger at "her" family. Especially now that you are a half a world away, you are the easy scapegoat.
I hope they do have family therapy. A good therapist will be able to unveil the process of drug abuse, identify the family systemic issues inherent in such a development, and help her/them accept the reality at her/their own pace. You should be exonerated by the end of this scenario, unless stepmum has far deeper issues.
Again, I preface this by assuming you were not involved in any nefarious way (abuse, modeling drug behavior, etc.). Otherwise, give them their therapy and allow them some time to recover from whatever "secrets" they are trying to protect by blaming you.
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