I guess for me it's sort of simple. I believe that it was a conspiracy the 4 of them cooked up. They took different roles but planned it together. Billy, Jake and George went to the 4 homes together and left together. They got home together and burned the clothes together.
What supports this for me is how much criming the family did from the time the kids were small. Theft, arson, burglary, drugs, frauds of various kinds. All of them, from Angela selling puppies from the pound to the others to burning their own house down, they stuck together, never turned anyone in. Jake's testimony was that he was taught how to hide evidence the way my dad taught me how to wash a car properly. These are criminals. It makes no sense that any of them would go out to murder 8 (or more, had anyone else been there) people without taking the others as backup, if nothing else. Really, there's not all that much evidence against JAKE, other than his confession. Without his testimony and whatever Angela told them, all the state would have is that 4 people planned this, carried it out, and covered it up.
And finally, I don't see George as all that much different than Jake in how he treated his wife, the selfish way he wanted to sever her ties with their child, and how he would allow his mother to bully and threaten her.