I made my fiance's sister cry
Why do something like that?
Her boyfriend is a junkie and she needed to find out sooner or later if she chooses to be with him.
It sounds like that is a problem in their relationship. Unless they bring you into their issues, you should try to be kind and stay out of the drama.
It's a part of my plan to end their relationship.
I can understand your concern for your fiance's sister, but I still think that you should let them handle their own affairs.  She has a right to know about her partner, but now that she does know, it is her decision whether or not to stay with him.  Unless she is in danger, you should leave her alone.