You decide to experiment. Well, the thought still scares you but also interests you a fair bit. This seems like a great way to know whether or not the cabinet is really magic.
You notice that you can pull out the tab that has the papers in the front of the drawer. You erase Sarah's name and replace it with Millie Roberts, your 10 year old cousin's name. Putting back the tab and opening the drawer, you notice everything changed, it now has Millie's information. Since she's visiting your apartment tomorrow, you decide to bring her into the experiment.
You decide to sleep since it would be more interesting to see the changes happen in real time.
By noon, Millie's mom has brought her to the apartment.
"Hi, John! I wanna play on your PC! Can I???"
"Sure, just let me open it up for you," you reply.
After waiting for your pc to boot and then opening Sims 4 after, you go over to the filing cabinet. Vicky is on the sofa watching TV, and Millie is on the PC playing, so it should be easy to observe the changes between the both of them.