The problem here is that the whole family is hunkered down there, the brother and their famous sister. They obviously do not think they are murderers.
While I of course don't know which way this will turn out... and all of this is just MHO...
There are some major problems with the SIL theory. It would be a complicated crime, drop off and then return un-noticed by your wife, leave no trace for LE to find. Every part of their story is easily checked. If they lied about anything so far, LE would be questioning them at the station, and their family would not be holding hands in those videos. They would need to be in on this together. They don't seem to have the technical background to evade the FBI in a bitcoin for body scheme. They might or would be expected to know the cameras have no subscription, so why destroy them. They cannot be sending ransom notes from that house.
Alternate explanations: FBI has set up communications tracking equipment in the home in case the kidnappers contacts them there. That is a lot of the cases we see going in and out. They are there as adjunct security for SG. They took pictures of the car to make sure it isn't any of the cars they are seeing on video. Some of the items from Nancy's home were removed and are now in Annie's home, like her purse, keys, phone and LE is examining them there. There is other communication with the kidnappers we are not privy to. I can go on and on.
