Ooops! I was sure I had posted this as a vanity.
First, if your BIL/SIL have a computer with an unsecured wireless network, it’s totally possible their computer is totally shared with any other computer who logs on to the network. At our office, some computers have a variety of folders which can be accessed, added to and deleted from, by other computers on the network.
It’s possible too, from what you’ve said, that your BIL has found a shortcut on his computer to the other computer (though why that should occur is strange). If your BIL’s computer is/was wholly “open” to another computer in the network, then it’s possible his neighbor has placed the photos on your BIL’s computer in order to avoid having them on his own computer.
Which means ...
See a good defense attorney FIRST. The few hundred bucks you spend is chicken feed compared to what your BIL will face if he’s fingered as the original acquirer of the porn.
Only after your BIL has gotten a good defense attorney should he and that attorney make a visit to the local DA. It’s entirely possible that they’ll want to monitor the situation to validate your BIL’s account of the situation.
What would BIL do if he found 5lbs of heroin in his garage?
1) He would toss it and forget about it, or;
2) He would tell the police to come get it.
The same thing here. Except do not by make a ‘secret’ copy of your hard drive and lie about it (as one poster has suggested). Lying to police in an investigation is obstruction of justice.
If he had called cops the immediately he would be better off. Now he will have to answer the nagging question of why he tampered with the HD - but he has a good answer: he was scared.
If the cops are interested they will take a declaration from your BIL so they can get a warrant to search the neighbor’s computer. Once they find the files on the neighbor’s computer they will have no interest in BIL.
Most lawyer’s will have no experience with child porn cases, fewer still have any tech knowledge, so there is no guarantee they will be helpful. The time for lawyers is if someone gets charged. Just don’t make any false confessions in the mean time :)