If she continues to push the issue of not being added to your already existing account, I don"t think I would have a problem telling her, "My first obligation is to protect my family, and I have to consider your financial track record". If she asks what is that suppose to mean, I would give her a long look and then say something like, "I am trying to work with you on this, but frankly, you have given me reason not to feel very safe".
She knows you are talking about all the debt and other trouble she has had with money. But she may goad you into a fight, so be careful.
