I've gotten to know a few people, here, who are raising children because the child lost a parent to addiction. Yes, it is tough on all involved, but I've seen several kids work through the grief and thrive because they are in a safer place, loved, and have support (which you have provided for your stepdaughter).
No, it's not what you would wish on anyone, especially a child, but I really do think you're doing a really awesome job with helping your stepdaughter through this.
Hugs and prayers,
Amy