Instructions
Create or play in a file with at least two Sims. This will prevent the game from ending when the Grim Reaper appears.
Kill one of your Sims. The method used to do this doesn't matter, though drowning tends to be the quickest. Tell your Sim to go swimming then remove the ladder. After a few minutes he will drown and the Grim Reaper will appear.
Pause the game and press "Ctrl+Shift+C." The cheat menu will appear. Type "boolProp TestingCheatsEnabled true" then press "Enter."
Unpause the game. Holding down the "Shift" key, click on the Grim Reaper and select "More," "Spawn," then "Tombstone of L and D." This will cause a tomb to appear on the floor.
Click on the tomb and select ""Add Neighbor To Family." Scroll through the list of neighbors and select "Grim Reaper." He will now be added to your family.