I would think IF the OSBI did suspect any family members they would not announce it, perhaps say just the oppisite, LE's would want them to feel they are cleared. In other cases they don't announce it's family unless they have already fled or have enough evidence for an arrest.
As far as "why would a family member have reason to murder" what kind of reason could ANYONE have to murder two young girls.
It happens...and it happened.
When this horrific murders happened I did not feel it was family, it's been four months and nobody has talked, I've been around both families and.....
I've changed my mind. Most locals have thought this from day one.
I don't have the statistics but child murders are often family/broken families or someone very close to the family or a sex offender.
I do hope I am wrong!!