I think he shot everyone in the family and set the fire hoping to incinerate all the evidence. However his brother's body somehow made it to the front lawn (he may have crawled on to the front lawn before dying), so the cops know a gun was involved.
I am betting there was an insurance policy.......................
I read the wife and 2 children were stabbed. the brother was shot in the head. whoever did this, it was a massacre.