More son-in-law than anything. Ashleigh Banfield said that a source close to the investigation told her that he was a prime suspect, and his car had been seized.
Here's the problem though, that's exactly what you'd expect. He and Savannah's sister were the last people confirmed to have seen Nancy, and they'd absolutely be looked at first.
You'd expect interviews, phone downloads, perhaps requests for DNA, etc. You'd also expect them to be asked to allow their car to be inspected, and a search warrant to be obtained if they declined (might get one anyway).
So I see this as pretty reckless reporting, as there's a good chance this is being overblown.
Would I be shocked if he was ultimately found to be involved? No. But the longer this goes on, the less likely that possibility seems.
Not sure what that structure is, but we've heard of no-one living on that property other than her.