his one is a little tough. I mean you have 3 kids of your own and usually this type of stuff doesn't happen to 16 year old kids and family? I mean you have some choices here.
1. You can go to the police and say the things her mother is doing to her.
2 Are you doing everything you can to let this mother know shes messing up. I mean not even joking or being afraid to offend her but are your boundaries clear. Often times we as adults don't set pricise and clear boundaries or if we do we are afraid to enforce them. If people know you are clear about your boundaries they will look for another weaker point.
3. This girl is 16 years old. Are you struggling to make ends meet? I mean i never raised a 16 year old (im a male) but I know how it is to be a 16 year old male. And she has no car, its a lot of work. So you do have a chance to make this girls world and show what love and caring is cause from the sounds of it, nobody has shown her it before.
4. If you are going to get advice remember its just advice. Those people aren't you. So they can suggest you are being used but that doesn't mean I'm going to take it for a 100% considering most people aren't as nice as me and don't really care as much about people to begin with.
5. And the most important how is this effecting you. Your self esteem, your self worth, your self respect. Is it making you insecure well if it is you need to work on your self and get your mental health up. Remember you have to take care of yourself and your family. If you want to make her part of your family you can do that but it sounds like shes on the outside anyway. Who cares about getting used. You can't die with your money or you house. But is it tearing your family apart, bringing your mental health down, or even worse a marriage (you never mentioned anything about a husband/boyfriend). And is she a friend or an unruly teenager. Those are some questions I didn't see.
Edit: you want my advice move the girl in and start teaching her and showing her the ways of life and teach her something. Shes 16 heck 2 years and she will be a young adult able to do what she wants in her life for the most part and she has no clue how to support a family or shes living house to house. Her mother set this poor girl up for failure. You could help her.