i had similar instances, myself as a child. both of my parents left me. i started stealing stuff, same age. in retrospect, it was more like a passive-aggressive behavior. i didn't really even want the stuff i was stealing. i wanted to hurt the people i was stealing from. cough drops, pencils, things like that. i was jealous of the people who had parents who "wanted" them.
M., you are doing such a beautiful thing , being so good to this little girl. one of the things that seems so important, from my perspective, is to make all the children that feel unwanted, left behind, abandoned... feel special. i never felt special until i met my husband. when i was 30. that included several different boyfriends, several children, drugs, violence.. well you get the picture. now, i'm not trying to give you a pity story, just to let you AND YOUR HUSBAND know you have the power to make sure this girl as well as her sisters are coddled at all the right times. i think it might be nice too if you talk to the girls about calling you mom, if everyone is good with that, and lose the step-crud!( if mom were here, i would have a different opinion) pls make sure dad touches all his girls in every appropriate way, this is how we learn to have good attention. finally, if you'd like to talk more, pls feel free to email me.god bless and good luck! M.
p.s. you know i was thinking about all this i wrote, and i wanted to add, part of loving the child is disciplining her! stealing or lying is absolutly not tolerated in my house. when i found my son had stolen something, i took his little buns right back to the person, and made him tell them what he had done.
i don't want you to think i meant coddle her for stealing!! who wants to live among liars or thieves? that turns a home into a house, to me.