How can I take over guardianship of my disable brother who lives in a rehabilitation home?

You can file a motion to open the case and remove her as the guardian.  You can ask to be appointed as the guardian instead and cite the reasons why she should not be allowed to conitnue to serve.  The paperwork for the guardianship is a public document and you can get it form the court in the county in which the guardianship was completed, most likely whereyour brother lives.  

If you need to amend the guardianship order, you will have to file an application in the court that granted the Order.  For example, if it was granted in Monmouth County, you apply to Monmouth County Surrogate's Court.  Call the County Surrogate's office in the county in which the person lives, and ask if they have a guardianship for that person.  The documents are public documents and you can get a copy of all of the papers at the Surrogate's Office.