Disclaimer: I am not a lawyer, but I am a writer.
Selling a story you wrote about characters you created in the Sims should be fine. You created the characters. You cannot name the setting “Willow Creek” or whatever, nor can you interact with Maxis townies without renaming them and changing them up a little. Basically, you can only use your characters and settings, not Maxis’.
The medium in which you created your characters doesn’t matter. They are yours.
People do this all the time. Fifty Shades of Gray was originally Twilight fanfic. Cassandra Clare used to write Harry Potter fanfic, and I can see some tropes and archetypes she used in both her fanfic and in the Mortal Instruments series but there’s no copyright infringement. It’s called “filing off the serial numbers.” Maxis owns Bella Goth, but they don’t own, say, a mysterious Latina in a little red dress married into an aristocratic family. You probably can’t have her get abducted by aliens without switching it up a lot, and your lawyer might prefer it if she wore a black dress instead, though.
If you’re instead only interested in writing for your own amusement, you don’t need to file off any serial numbers. Just say “Bella Goth” and “Willow Creek” and move on and feel free to slap it up on Tumblr or AO3. But if you want to sell it, you do need to rename and tweak anything that belongs to Maxis. But not the characters you created, because those are yours.
Now, images… published stories for pay generally hire a paid illustrator. I’m less clear on selling screenshots because my focus is on text, lol. But if you decide to sell it later there’s nothing wrong with pulling it and editing it and removing screenshots, etc. The characters remain yours.
