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I am so sorry for the pain this addict has caused to her children. You are there for the SD and give her a safe home.She is blessed to have that. (I feel sorry for her step brother) Not sure if you could recommend alateen for her. This is kids that are dealing with an addict in their lives. This might be helpful for her to understand her alcoholic parent. I am sure someone on this forum can recommend a book or something to help her.
All you can do is be the best step mom you can be. She is blessed to have you in her life to love and care for her. Hugs my friend, she will be ok with her "parents" support and guidance.