Good morning, T.,
Fathers are very important to a child's identity. Fathers were created to show boys how to become men and daughters how to be treated as a woman by the way they treat their mothers. The stealing is indeed a cry for help.
So make sure that your husband takes all his daughters out on a regular basis, including yours. Their should be a "date night," as in do something that both want to do, be it movies, dinner, whatever . . . just time between "Daddy and me." And while you are at it, incorporate a family game night, or something, where no one else interferes with family time.
Treat her like you would treat "your child," meaning that "your child" and "his children" are no longer your children individually. They are your children together. That means that you care for her in the same fashion. So you and hubby need to set down some rules together concerning all your children. Just because you did not birth yourself does not make her any less than your child. So don't treat her any different. The best thing you and your hubbby can do is to give her a stable and loving environment that she can come home to, where there are clear rules.
The situation with her biological mother will not change and until she deal with her own issues. And she will not get the moral training she needs from her until Mom comes to grips with her own moral issues. So don't expect it until the Lord brings her to the place of healing and wholeness.
Now, I don't know if you have a relationship with the Lord, but you will need the Lord to help you navigate through this. Only HE can help you ultimately. All we can do is give you advise on how to proceed. So as with any and everything, cover in prayer. Only God can speak to man's hearts. Only God can reach to the interiors of a person soul.
In all your getting advice, you will need God's direction on what will work for each of your three children.
May the Lord help you through this time.
One last thing, if you and your husband have not sat down and talked through the issues of how to handle the children, then now is a good time to do it. Remind him that all the children are yours and that together each issue must be dealt with.
There indeed alot of unique issues in a "blended family." But by the grace of God, you will make it through.