Can I safeguard my mother even if I'm not designated her guardian?

First, be sure your brother has true legal authority to care for your mom. If he has been appointed her conservator, which is the name for a guardian of an adult, it means that a court has determined that your mother is incapable of managing her own financial or personal affairs, or both--and that your brother has the legal authority to make those necessary decisions for her.

If this is the case, first attempt to change your brother's approach by having a heart to heart talk with him--simply underscoring that you are concerned about your mother's care and well-being. Come to the conversation equipped with some specific concerns you have--along with some possible solutions for dealing with them differently. If this isn't possible, or your pleas fall on deaf ears, then your strongest approach would be to have your brother removed as conservator. You will need to go to court to do this--and will likely need to hire a lawyer to get the job done.

However, if your brother is not your mother's legal conservator, but is acting under the authority of a less drastic arrangement, such as a durable power of attorney for healthcare or finances, you can investigate to make sure he is not acting beyond the power these documents grant him. And if he is, make that clear to the medical personnel or financial institutions with which he is dealing.

And finally, if your brother is not acting under any real legal authority, but is just overbearing, you may be able to help out by being authorized to handle her finances in a power or attorney for finances or to handle her medical care in a power of attorney for healthcare. For this to happen, however, your mother must be of sound mind and willing to name you to act for her in the event she would become unable to do so for herself.