What documentation do I need to enable the companies to discuss her accounts with me?
There are two routes for you to follow. The first is to contact each bank, company and other entity your mother has accounts with. Many banks and other companies have their own form that they would want your mother to fill out and sign, which would allow you access to her information. Many businesses ask that their own forms be completed regardless of what other legal authority you have, either to discuss or conduct your mother's affairs.
The other thing you can do is to prepare, with your mother, a power of attorney for finances. This could give you the authority not only to discuss your mother's accounts but also to conduct business on her behalf (if she wants you to). The extent of the authority your mother grants to you, and when it becomes effective, is up to her. For example, she might grant you only limited authority to discuss her financial matters now, with you getting full control over her financial affairs if and when she becomes physically or mentally unable to make decisions for herself.
