My sincere condolences to your family on this loss.

As close family members, the siblings can demand an autopsy is performed.

This is the statute that controls coroner actions in Georgia:

45-16-24 G
*** CODE SECTION *** 12/03/01
    
45-16-24.
    
(a) When any person dies in any county in this state:
    
    (1) As a result of violence;
    
    (2) By suicide or casualty;
    
    (3) Suddenly when in apparent good health;
    
    (4) When unattended by a physician;
    
    (5) In any suspicious or unusual manner, with particular attention
    to those persons 16 years of age and under;
    
    (6) After birth but before seven years of age if the death is
    unexpected or unexplained;
    
    (7) As a result of an execution carried out pursuant to the
    imposition of the death penalty under Article 2 of Chapter 10 of
    Title 17;
    
    (8) When an inmate of a state hospital or a state, county, or city
    penal institution; or
    
    (9) After having been admitted to a hospital in an unconscious
    state and without regaining consciousness within 24 hours of
    admission,
    
it shall be the duty of any law enforcement officer or other person
having knowledge of such death to notify immediately the coroner or
county medical examiner of the county wherein the body is found or
death occurs. For the purposes of this Code section, no person shall
be deemed to have died unattended when the death occurred while the
person was a patient of a hospice licensed under Article 9 of
Chapter 7 of Title 31.
    
(b) A coroner or county medical examiner who is notified of a death
pursuant to subsection (a) of this Code section shall order a
medical examiner's inquiry of that death.
    
(c) Whenever an affidavit is made and filed with a court having
criminal jurisdiction attesting that a person came to his death by
foul play, that court may interrogate and examine witnesses, if any
exist, as to the necessity of a medical examiner's inquiry. Should
the court decide that a medical examiner's inquiry is essential to
the ends of justice, such inquiry shall be ordered by that court.
    
(d) A medical examiner's inquiry required under this Code section
shall be reduced to writing and filed as provided in Code Section
45-16-32. At the time of such filing, a copy of the medical
examiner's inquiry into a death reported to a coroner or county
medical examiner pursuant to paragraph (6) of subsection (a) of this
Code section shall also be transmitted to the department of family
and children services of the county in which the child resided at
the time of death.


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I suggest the siblings contact the county in which your brother-in-law passed away, and state that as per section 5 of the above mentioned code, you believe the passing to have been suspicious, and ask to have an autopsy performed.

Sincerely,

Dimitry Alexander Kaplun, Esq.