If you want to sever your ties with this person and you can afford to pay what she has asked of you, then pay it and be done with it.
I don't think you will ever get the money which is owed to you, and you will be happier if you realize that, and try to make better decisions in the future about who you purchase anything from as well as who you agree to "take in" to live with you.
If, on the other hand, what she is asking of you is more than you think should be asked or if you cannot pay it, then tell her you are not going to pay it because: then list the things you did for her nephews etc.