No wonder that this young girl has abandonment issues, her mother has abandoned her! She needs you. Please don't give up on her. She needs you and her dad to take charge and help her through this abandonment and to build a healthy family life in your home.
I have been a stepparent for 26 years (kids were 1 and 4). My stepson was abandoned on his 14th birthday by his mother. His stepfather dropped him at the sidewalk with one bag outside the airport and he came to live with us. We were in family counseling for a year (26 counselor visits) to help him deal with this. It worked. After a year of his mother choosing not to speakto him - not once - we worked through his feelings of anger, frustration, hurt (he was abused by his SF and mother) and all the rest. We then got to enjoy his teenage years.
All the pain and struggle was worth it. He is now a wonderful husband and father to three young children. He has a distant relationship with his mother now and a close one with me.
You may not have peace in your home for awhile. You'll have to find moments of peace in your heart, knowing that a young person relies on you (but may never admit it or thank you for it) and that you are doing what you can for her and for your family. Find a counselor that you feel comfortable with, who can help you and your husband not get lost in this, who can give you concrete actions and parenting advice, who can relate to a young tween going through a lot.
My family life was not peaceful often, but I would do it again for both my stepchildren and the joy they have brought me along the way. I now get to be "Grandma S." to their three, soon to be four, beautiful children.