When Jacob enters his fathers house after his funeral he finds the knife his father took his own life with. Both the knife and the carpet in the room where the death happened are covered in fresh blood. Since the funeral took place at least one, possibly more days after his fathers death there neither wouldn't be fresh blood stains nor the knife itself. It'd be a crime scene and therefore totally occupied by the police.
There are no legal requirements for parents to leave any inheritance to siblings in any state in the United States. So unless the aunt were to challenge the will (which is impossible as she is deceased) there would be no need to find this person as they would have no legal claim to any assets .
The goof item below may give away important plot points.
Death certificates are records which are held by county or state governments. If someone is declared to be deceased by a medical facility, then the facility needs to file that documentation with the county or state as soon as reasonably possible. That means that someone who is looking for a death certificate could easily go to a state office and receive a copy of that certificate, within minutes if they are a relative or someone with power of attorney.
The attorney would have known this, rendering the entire premise of the film incorrect.