Anonymous Letters Wont Go Away

Likely to be someone who knows the family well, given they have sent them to workplaces, especially the workplace of the one with the new job. There was a film with Olivia Coleman with this scenario. Interesting how it panned out. It's called Wicked Little Letters.

I agree with those saying not to bother opening them. Keep them in a drawer or bin them, but there's no need to have anyone getting distressed further by any recipient reading and sharing the content of any further letters.
 
You have 3 choices here, one is to ignore them as some posters state but given the circle has been broadened and the fact that it is clearly someone you know, that will only fester doubt, paranoia and suspicion in your mind.

Secondly, you can try and catch them yourselves, and I would agree with what some posters are saying regarding cameras. If you do get evidence, don't confront the person, go to the Gardai

Thirdly, make a formal complaint to the Gardai and ask for a report number. Then get the rest of the recipiants to do likewise. Gardai saying there is "not much they can do" sounds more like, "can't be bothered, too difficult." A whole pile of complaints are more difficult to ignore

Do the letters make any specific threats?, regardless, I'd be stressing to the Gardai about the stress and fear this is putting all the recipiants under.
 
I would have to say based on the information you've given us here that your mate knows this person. Hand delivered means they're local enough. I also think there's a family grievance rather than just against you, considering the older brother is involved.

The new job letter is your key. If they're bothered finding it out, trace who could know.

My money is on discovering its a family relative or neighbour.
 
This happened a friend of the family a few years ago and it went on for many years, it caused great stress and anxiety, they had no idea who was doing it, but the letter writer was caught out in the silliest way, it was a "close" sister of the victim and they saw a letter already addressed and stamped in their handbag during a bathroom visit. Victim couldn't believe it was someone so close and the sisters never spoke again.
 
Likely to be someone who knows the family well, given they have sent them to workplaces, especially the workplace of the one with the new job.
But maybe they don't know where the family members live and are using social media to find where they work instead.
 
If they used LinkedLn, the new people should be able to check the 'Who's viewed your profile" without getting Primary and see if there's a match on LinkedLn. Even if they set up their privacy it will still give a clue such as some from 'dodge crop' viewed your profile. The job should be enough to work it out.