Yes. She has to sign in with your account. You can't register it on a second one.
Saves are saved locally (no cloud saving) so she'll have her own on your mac.
Downside: Only one user can be online at the same time.
Let's put it this way.
Only one person can be online at the same time.
You can play Sims 4 in offline mode.
But how would I go about logging into Origin though as that tends to need an internet connection?
It does say I can play offline but could I disconnect my internet for instance, then play in offline mode and then we could be playing the same game at the same time, in the same account without issue? I thought it would keep you out of the account if I tried to log in now (she is currently in my account downloading all the games now) that it wouldn't allow me to?
If you could elaborate that would be awesome!
You can login to Origin, then put Origin in offline mode.
So yes, you need to login into Origin but that can be done offline
While one is online, no one else can login.
