You definitely need to seek professional help for your step-daughter and family therapy for the entire family, including yourself and your husband and if possible the birth-mother. At the tender age of 8 years this child has seen unpleasantness, separation and drug abuse by her mother. She is bound to be traumatised and confused. As a step-parent, you can only provide unconditional support, warmth, acceptance and keep communication channels open. But continuous individual therapy for the child is strongly recommended.