If you want to pretend that Agnes died, there is a way. You will have to keep her name, but you can make her a different sim.
All you have to do is access cheats (Ctrl+Shift+c to open dialog window, then type testingcheats true) and then open the cheat panel again and type in cas.fulleditmode and then after that shift-click on Agnes in the game, and choose Modify in CAS to open CAS. Then click on the light bulb in the upper right corner to open the gallery. Choose My Library or or the Gallery and click on any sim you want to replace the old Agnes. Click Add Household. Change the sims name to Agnes Crumplebottom. Then delete the old Agnes by clicking on the old sim you no longer want and then click on the red X. You can then choose another sim to replace Agatha in the same way, also changing the name to Agatha Crumplebottom. Then click Play to return to the game. Choose your sim's household from the map and rejoin your sim in town. The stand in town will be closed. Wait a minute or two and either the new Agnes or the new Agatha will run over and open the stand.
The new sims will still do the same thing the old sims would have done. Your new Agnes will still get angry a lot and be mean to other sims, and so forth. But they will be a different sim and you can pretend the old ones died.