What authority/power is given for guardianship over someone?

In Nevada there are a total of three levels of guardianship.
1. Guardianship for the protected person. Requiring the guardian to be held responsible for food, shelter, personal needs.
2. Guardianship of he protected persons estate. Meaning the protected persons finances while the protected person is still alive.
3. Guardianship over the protected person and the finances of the protected person.
It can be contested by almost any one. But our courts here lean more to the family
being granted guardianship.
It does require a court appearance and the declartion of incompetence of the protected person, which must first be diagnosed by at least one doctor.
DW's lawyer appeared in court with me to represent her and supported my request for guardianship.
Restrictions mandated by state law were explained to me by the judge as well as the lawyer.
The only real restrictions I had were found in the state laws.
Record keeping and annual reports to the courts were required and limitations on how much money I was allowed to spend on a monthly basis. Any amount over that required prior approval by the court.
And we lived happily together until the end.
God luck to you.

He will have total authority (I'm not sure if it differs from state to state). But you must go to court to prove why you are seeking guardianship. FYI I think someone can contest it or be a witness for his brother. Other legal eagles will post more detailed info here shortly.

Complete decision-making authority, but subject to the scrutiny of the court which awards guardianship, to which your husband will have to submit regular reports and accounts.

And, alluding to the situation I remember you're facing, it doesn't mean that your husband can't be challenged if those caring for your BIL believe that your husband's decisions are not in your BIL's best interests.

So it won't solve everything, but it will hugely improve DH's and your access to information about your BIL.