Who REALLY did this???

Ok, let me start off by saying that I have followed this case for 20+ years and have ALWAYS thought that the Family did it. It doesn't matter who in my theory, just follow along for a second. No matter who it was, how does the SA come into play? That is the one thing that I could never piece together. Why go so over the top in "staging" that if a family member did it? The only plausible answer to me is that the SA had happened previously and was discovered after the fact. Again, let me state loud and clear, that I think someone in that house did it. I just can't figure out why/where the SA staging could come into play. Discuss!