It probably will get harder as time goes on, and realisation sets in, but at least she has the stability of a loving home to carry her through any dark days in her future.
She is so lucky to have you there to support her during these hard times.
When I was her age, I wanted my father to stop drinking, tell me he was sorry, tell me he was proud of me, and most of all tell me he loved me. He never did.
You can tell her this is just out of her bio mom's realm of feelings, her feelings are gone except for her alcohol and how that effects her moods. Anything she says that is lovey dovey is hooey to make a show. Let stepdaughter stay away, and not play that part that mom has for her. Make her the star in her own life, and the need for mom will lessen, not really ever go away, but she has you and your husband there to take up the slack. You are working so hard on that. I appreciate it, even if the 13 year old can't say it now, I will. Thank you for being there for her and her brother.
Beth