I found it, but I'll post my solution here, in case others may have the same problem.
I just entered a new quest, where my Wizard would be the hero. Then I let her go to the trone room, because there I can click on the chest to change her outfit. I entered the Create a Sim mode. I took a nice screenshot (key C) as a reference, and in the panel on the bottom left was a tumbnail of her face. When hovering over is, her full name appeared.
Now I can create her twinsister the Physician!
Since this wasn't the quest I originally wanted to play, I just quit without saving.