One of the questions I get (unfortunately) goes something like this: “my sister has persuaded Mom to put her on the bank account and now I can't get any information about what's happening with Mom's money. Sis is telling Mom not to talk to me about anything. What can I do?”
The answer hinges on whether the elder is competent to make financial decisions — that is, her ability to understand the possible ramifications of a given financial transaction or business arrangement. Lack of competence is not the same as making a decision about which you might disagree. In the case of adding a child to a bank account, competence means whether Mom is able to understand that adding the child to the bank account exposes the funds in that account to the child's creditors, that Massachusetts law treats the action as making a lifetime gift of an asset that would otherwise go through probate, or that the child could legally walk off with all the money at any time.
If Mom is competent, then there may be nothing which can be done except to spell out to her the ramifications of her action and see if she'll change her mind. Competent people have the right to make bad decisions. Mom may be afraid of Bad Daughter, but as in any abusive relationship, Mom has to decide for herself if she wants to take the steps needed to detach from the abuser. Elder Protective Services can provide assistance if Mom wants it; but if Mom is competent does not want their help, they have no legal right to act. Alternately, you might want to hire a geriatric care manager to provide Mom with nonjudgmental professional support, which could in turn help Mom develop the courage she needs to regain control over her funds.
If you believe that Mom isn't competent, Elder Protective Services may be able intervene. If that is the case, the agency will seek protective orders asking for a competency evaluation and possibly seek to have a conservator (who may or may not be a family member) appointed. If you decide to bring a conservatorship action, and if Mom refuses to go to a doctor for an evaluation of her competency (or Bad Daughter interferes with that effort), then you can ask the judge for an order for an evaluation and another order to freeze Mom's assets. However, be mindful that this step can inflame existing tensions. You need to decide for yourself if you are willing to live with those consequences.
To find a protective services agency in your Massachusetts community, go to http://contactus.800ageinfo.com/FindAgency.aspx