I doubt these cases ever get solved, it’s going to take a confession/informant and that’s not likely to happen. Whatever the motive is, they are smart and know how to ambush, plan, remotely isolate targets with police knowledge of how to cover their tracks. If all of that is true then they won’t get careless mistakes from a dumb drug dealer etc....so they won’t get a dumb criminal looking to trade info because they got busted for another crime.
Of the three murders Crystals has the best chance of being solved due to a chance of evidence, her fathers murder and Ellis have almost no chance of producing evidence unless they find a shell casing at the fathers crime scene. Of course if the mother and daughter murders are related to the other crimes then they become the best chance to solve all four because it happened in their home which should have more potential evidence.