Your stepdaughter is a lucky girl to have you and your husband. Thank you for doing right by her.
I adopted two children from foster care at ages 8 and 9. Their mother was severely mentally ill and they took care of her. They worried so much that she would die without them being there. They were what we call very "parentified." My daughter had been managing the family's finances since the age of 6!!!
We just kept repeating to them that adults are there to care for children and not the other way around. And we often had to stop them from doing something parental (worrying about whether we had enough groceries, whether the power bill was paid up, weird stuff) by repeating over and over "I am the parent. My job is to take care of YOU" or "It's my job to worry about that stuff --- your job is to be a kid!"
I think it's so great that you and your husband are being honest and straightforward with her. She needs information and constant reinforcement that none of this is her fault and she has zero control over her mum.
Counseling with a therapist who has experience with addiction and attachment issues would be a great idea for your daughter.
My now teens are dealing with their adoptive parents separating due to my husband's alcoholism. I have repeated the 3 C's to them often (you didn't cause it, you can't control it, you can't cure it). They say them back to me when they are having a hard time, and it really seems to help them frame their situation in a way they can understand.
Hugs to you and bless you for being such a good mom.