As our parents age, it is natural for us to think about who will step in to make their financial and medical decisions when they are no longer able to do so. This becomes much more of a concern if they are diagnosed with dementia or Alzheimer's disease. The best course of action to take is to make sure that your parents have a financial and a medical power of attorney in place so that the person they choose to make their decisions will be able to step in when appropriate.
However, this is not a possibility if a parent has already become incapacitated, no longer able to make decisions for himself or herself. In this case, you may be able to make some financial and medical decisions for your parent. Most likely, you will run into road blocks from banks and other financial institutions and from medical providers. They have a legal duty to protect their client's (your parent's) privacy and property. When this happens the only option may be to seek guardianship.