Sorry, but it's not rude nor is it illegal.
The only way(s) the picture stealing would be illegal is if the nephew is passing it off to make a profit. He would be selling someone else's work as his own. Another way is if your DH was a professional photographer and took the photos professionally, was paid for them, and nephew tried to pass them off as his own work.
That is not correct. DH is a pretty decent non-professional photographer. He holds the copyright to his artwork just like any other artist. Someone copying his work and using it is in violation, whether or not they give him credit and whether or not they make a profit from the use of his artwork.
The thing with the internet is it is public domain. It's like posting a photo up on a bulletion board at work or at the local gathering place. But if someone is selling it in their name and making a profit off of it, then thats illegal.
Putting the photos on the internet does not negate the copyright holder's rights. That being said, FB does get some rights to the photo if you publish it there-which means posters really should be aware of the rights FB has to your photos if you post them there.
It does not matter how they are placed for viewing (internet, gallery, passed out at a club meeting, put into an art show), copyright is not negated just because the photos are available for viewing....no matter how easy it is to steal them.
All that being said, I find scenarios 1 & 2 only slightly rude. These were family snapshots, posted to FB to be shared. The original photographers, if I am reading correctly, have passed away-so the rights have probably passed down to the assorted family members.
Had I wanted to use those photos, I would have let the person know I was doing so and would have given credit for the work of scanning and fixing the photos for internet publication.
The third scenario, I find rude. Since your DH took those photos-he is the direct owner of all rights to the artowrk. Using someone else's artwork like that-without permission-is a violation of copyright...which means that it is stealing. No matter how easy it is, it is stealing. And stealing is rude.
Now-as, again, the photos are family snapshots, I would be inclined to either overlook the offense or-at most-ask to be given credit for the photo. But that does not mean that no offense happened. It just means that I would not find it a major offense.