That thought did cross my mind but what are the odds that the biological dad would ALSO be into the 'recording fad'. Setting up 8mm or 16mm film about the house just starts to make it all seem contrived. Even then the girls would likely be infants or toddlers who would the action center on then Julie? Lois babysitting when the coven drops by?
Any further back pre-70s and there just isn't a commercially viable recording system to continue with the saga's gimmick.
As for moving forward I can't see how you don't bring in a third party investigating the deaths that occurred in pt.2. With our media savvy culture that follows missing baby cases of the bizarre this could work easily and in a believable manner, imo.