One of the sims in my town has seemingly disappeared. The sim shows up with a blank portrait in relationship panels and family trees, but doesn't exist in town. I've tried resetting everything, resetting the sim using the cheat command, and using MC to try to invite her over, but her name doesn't show up in any list of sims that I can do anything to via MC. I have WA but none of the households in this town have ever travelled.

Not sure what happened to her; she had a twin sister that SP just aged up into a teen, so I don't know if something went wrong in the age up process or what . Is there any other MC command I might be able to use to recover her?

Sometimes, if a sim is "out of world" or hibernating they won't be available to be invited over, so that might be why you aren't finding them. Maybe try MC's Add Sim instead, and if they don't show up in the regular lists in MC, try "Type of sim (and/or)" and check some of those filters, especially Local Mini Sim. I've found a missing sim hiding in there before.

Some other things that have worked on various wonky sims was DebugEnabler's Fix Homeworld and Fix Invisible Sims, and Overwatch's Clean Up Outfits.