OP,
You may say that this BIL traipsing around you house, alone, and even having open access to your computers is only 'annoying'.
But, seriously, you need to maintain some stronger boundaries.
I can really see no valid or good reason for this BIL's behavior.
Trust, or not, nobody has unfettered access to my computer, my personal information, and even your husband's work....
After reading your updated post about your BIL, something sounds like it is really OFF. Does he have social deficits/disabilty, like Asperger's or something.
That would help explain it. And perhaps ease some of the concerns we are trying to bring to your attention. But, that does not excuse it in any way.
If this guy does not have anything like this, then really, I agree that it is even more than creepy to have a grown man traipsing around your house alone, wanting to access your computers, confronting your children. Really, something just isn't right here.