As a fiduciary of my incompetent brother who is a 100% disabled Vet, am I entitled to a fee from his estate?

If you are a court-appointed Guardian of the estate, the court decides what fee you will be paid and how many hours you are entitled to.
Guardianship ends at death.
VA Service center will determine whether a commission (fee) is allowed for a fiduciary when they appoint him/her. In some circumstances where no relatives are available to serve for free, the VA appoints a professional fiduciary and pays them a standard fee.
Once your brother dies, his Will becomes effective and the Executor handles the estate. Again the court decides how the Executor is paid, either a percentage or billable hours depending on how complex the estate is.