Thank you for your question.
It's not really accurate to say an executor of an estate
has power over another family member, because that isn't the case.
First, in order to be formally appointed as executor, a person must petition the court - merely being named in a Will doesn't give the person the power to do anything or speak for the estate. Second, the executor has a fiduciary duty to all the beneficiaries named in the Will. So, an executor cannot for example, drag their feet and probating the estate without good cause, or spend the resources of the estate in a manner not provided by the Will or take other actions that could negatively impact what the beneficiaries are entitled to.
If a probate
has not yet been filed, any interested party (such as yourself) could petition the court and ask that they be named executor. Granted, the court would give deference to the person named in the Will, but if you could show cause as to why you should be appointed instead of your brother, the court may appoint you. If a probate has already been filed, and your brother simply isn't acting in the best interest of the estate, you can petition the court for a formal accounting and request that he be removed and another executor appointed.
With respect to your mother, as he is the guardian over her estate and affairs, he has a lot of power to essentially act on her behalf. That said, however, here again, he has a fiduciary duty to act in her best interest. He couldn't, for example, take her money and use it to buy a car for himself. That would be a breach of his duties and he could be liable for such actions.