I am referring to guardianship and control over affairs.
OK. That raises a few more questions. I don't want to make assumptions that lead to erroneous information.
So let's define a few terms.
Power of Attorney - Generally means that your mother would have signed a paper giving your brother the authority to act in her behalf in a variety of ways, i.e. financially and/or medically.
Guardianship - Generally done by court order when the protected person is no longer competent, either physically or mentally, to handle her own affairs.
Was the guardianship of your mother awarded to your brother as a result of a court procedure because she was no longer able to handle her own affairs? It's also possible that she consented to the guardianship and it went through court to make it official so that he had greater authority than just POA.
If so, then he does have full control over her affairs but he also has a statutory and fiduciary duty to act in her best interests and not in his own.
Here's a handbook about guardianship in Virginia published by the Virginia Guardianship Association. Might help you understand it better.
http://www.vda.virginia.gov/pdfdocs/Guardbook.pdf
It would help to know if your brother petitioned the court to award him involuntary guardianship because of a mental or physical condition or whether it was something your mother agreed to.
If you want to monitor the guardianship I suggest you start by going to the courthouse and buying yourself a complete copy of the guardianship case file.
Now, do other family members (children) have legal rights pertaining to the estate?
As long as your mother is alive the answer to that is no, because her assets belong to her, to do with as she wishes, or if she is incapacitated, for your brother to administer in her behalf.
Upon her death, the guardianship and the POA terminate and the will takes over. Any "rights" to her estate that you and your siblings might have will be those that are specified in her will. As executor your brother would have to comply with the terms of the will and probate the estate accordingly.
As executor he would also have statutory and fiduciary duties with regard to probating the estate.
Here's a brief guide to probate in Virginia.
http://www.courts.state.va.us/courts/circuit/resources/probate_in_virginia.pdf