There is so much we don't know. We don't know where Borce and Karen were meant to be that day at 10am, or if home was where they were expected, where Sarah was, and what went on the night before. Was the family's digital footprint the night before consistent with their usual pattern, and with what they said they were doing? Who was the last person to see Karen aside from Borce and who last spoke to her on the phone? The police have been very quiet on the timeline, unlike in Allison Bade Clay's where we knew all about the hairdresser and so on. They'd have so much of a better idea of what went down.
I think this case will crack when Sarah does. If she does. I think they are all well versed in pretending things are different to what they really are, so she may not.