After the death of her sister, a suicidal superheroine's drug addiction grows as her family fights to stay together.

I think you need to ask yourself, "what's at stake when this superheroine is no longer on the street doing superhero stuff?" What I find interesting about this concept is a superheroine using drugs to escape the reality of having lost something. It might have a greater impact if what she lost was her daughter, rather than her sister.

You might also ask yourself the question, "What happens when she uses drugs?" Do drugs make her normal or do drugs enhance some superhero ability? For better or for worse?

You might find that by asking yourself these questions you come up with new expressions of your logline that are more compelling.