I don't see that there is any large inheritance to be had. The land is mostly tied up in et. al. w/the other brothers. The rest will most likely have to be sold to pay funeral bills. Neither of the married partners killed the other for freedom from a marriage, custody disputes, or a life insurance policy.
None of the other family members seemed to have life insurance policies to kill over.
FR and his three year old son's mother, seemed to have custody/visitation worked out and there's a very minimal inheritance for him.
There seemed to be no issues surrounding FR's and HG's infant (we don't have dna proof yet, though), and there's very minimal inheritance for him too.
From all we know, pot is the only illegal activity. They didn't seem to be big enough players though, to be killed over pot.
HR's infant has parental issues. However, no need to murder an entire family over something that was not yet known, especially if you are already the father of one child, and have no custody/visitation issues surrounding the first child. The other, I don't see murdering his own sibling over something he did not yet know. I just don't see either man, in the picture, murdering an entire family over a dna test that they didn't yet know the results. Not even if they did know. It seemed the family was very good w/working things like that out. Unless you are a potential father, who is known only to the family, and doesn't want to be outed. Ever. Still, that's a very slim likelihood.
Just trying to hash out things I'd think would NOT be reasons for someone to wipe out an entire family. For me, there's not a reason to kill an entire family but, just trying to think outside the box...