Then they have the right to request an autopsy......that's pretty simple
Why would the family agree to an autopsy knowing it would come back as murder by the family?
Think about it.
Uncle is 70, he dies over night after being poisoned by the family.
You state it should be up to the family. Well of course the family is not going to do an autopsy are they?
So they get away with murder and that is why law enforcement and judges decide not the family who decides.