This step-by-step guide is created for those who want the “Et tu...?” achievement. As you know, since patch 16.1 the mechanics of killing family members have been changed and because of this, this achievement has become unattainable.

We are talking about this button:


Player requests to Creative Assembly for a fix are ignored[community.creative-assembly.com].

This guide is largely based on the guide on downgrading the game, and therefore I will refer to it in some places.

and on the discussion of this problem here:
https://cs2bus.com/app/214950/discussions/0/6553383999312515615

I am grateful to all the author of the downgrade guide Viscount Danku and all the participants in that discussion, especially JMTAotP, Pepe

1. Make a backup of your current version of the game in a separate folder
   
   
2. Download the necessary patch of the game in parts and replace your current version of the game
   
   How to do this is described in detail here. I can only add that the current version of the program now has an .exe file and you only need to run the following command from the terminal (from the folder where this DepotDownloader.exe file is located):
   
   DepotDownloader -app 214950 -depot 214951 -manifest 6611980476241080704 -username <your-username> -password <your-password>
   
   and so on for all depots listed in the guide.
   
   Attention!
   There may be an error while downloading depot 214951 (launcher)
   This is normal, just ignore and download the following depots
   
   
3. In case you as well as I failed to download the launcher, then copy the launcer folder from your backup version to the folder with your build
   
   
4. Turn off the internet and (possibly) change the date on your computer
   
   Disconnecting the Internet you need to ensure that the new launcher, which you transferred from the current version of the game, does not begin to synchronize game files.
   
   (not sure) I changed the date to combine getting the achievements from the “Make war not love” series. I changed the date to February 14, 2015.
   
   If you find out that this step is not required, please post, I will remove it
   
   
5. Run the game from the folder
   \steamapps\common\Total War Rome II\Rome2.exe
   
   If everything worked out at this stage - you've done 90% of the work
   
   
6. Get the in-game achievement
   
   Start a new campaign.
   
   I chose the Iceni because they have several family members at once at launch
   
   
   
   Go to your faction's menu
   
   
   
   Pick a family member and kill them (I killed all of them, just to be sure)
   
   
   
   End the turn.
   
   You may now exit the game. Do it through the main menu
   
   I got the achievement the moment I logged out of the game.
   
   
7. Turn off Steam, return the date, turn on the internet, start Steam again
   
   This sequence of actions led me to a result:
   
   

This is a detailed algorithm of what I did to get this achievement. Probably some of these steps are redundant (changing the date). If you figured it out - please post in the comments and I will make changes.

Apparently, achievement bugs are a hallmark of the Total war series. Unfortunately, Total War: Shogun 2 game has such problems and the developers are not in a hurry to fix them.

Keep this in mind in the future.