This is all about getting married in Sims 3, why your Sims might take the "wrong" last name, and how to fix it!  

First of all, if you are having trouble getting your Sims to the proposal stage, read my article Why Can't My Sims Get Married?  This covers the basics.  As a quick summary: spam with Romantic options until they fall in love with you!  But please don't do that in real life.  It doesn't work.  Ask me how I know.

The last name thing was an issue in the first and second Sims games, but many players are starting from scratch with Sims 3, not having played either Sims 1 or Sims 2.  KIDS THESE DAYS OMG I AM SO OLD.  

Okay, anyway!

Let's say you have two Sims named Kara Thrace and Saul Tigh, and you want them to get married.  (Hee!  I know someone who did this.)  Once they get married, you want them to be the Thrace family - in other words, you want Saul to take Kara's last name.  

At some point, the menu option "Propose Marriage" will appear on the Romantic menu for both Sims. 

1. Click to control Kara Thrace. 
2. Then click on Saul and choose "Propose Marriage."

The Sim that you are controlling when you propose marriage, is the one whose last name will be used by the other Sim.

When it is actually time for the wedding to start, do the same thing.  Click to control Kara Thrace.  Then click on Saul and choose "Get Married."  

Tada!  When the families merge, you will now find yourself controlling "The Thrace Family," with Kara Thrace and Saul Thrace.

Unfortunately, mistakes happen to the best of us!  It's easy to get caught up in gameplay, and accidentally control the wrong Sim when you propose marriage.  In The Sims and The Sims 2, if you were controlling the wrong Sim when you proposed, then you were SOL.  Your only option was to exit the game without saving.   

Luckily, The Sims 3 makes it easy for you to change your Sims' last names if you mess up.  City Hall offers you the option to file a name change proposal with the courts, just like in real life!  

Let's say you accidentally changed Starbuck's name to Kara Tigh, and you are now playing "The Tigh Family." 

1. First, select Kara
2. Then switch to the map view and click on City Hall. 
3. Choose "Change Name," and change her last name to Thrace. 

Then select Saul, switch to map view, click on City Hall, and click to change his name to Thrace.  

One trick about changing names in The Sims 3 is that City Hall is only open during normal business hours.  (I think these are 8AM to 6PM but I'd have to check that.)  If you click on City Hall at night, you will not have the option to Change Name.  You will have to wait until the morning.

When I had to do this name change swap, I found that the game did not update "The X Family" description for the lot.  I'm not sure if that's a bug, or what.  I switched to Edit Town mode, and edited the lot's family name from there.