Please see the first post of this thread about saves and other user data disappearing:
https://answers.ea.com/t5/PC/CURRENT-ISSUE-Sims-4-saves-user-content-disappearing-PC/td-p/12475370
The fact that you and your sister use the same EA account has nothing to do with this. Neither does any problem with the EA App itself, although the fact that the App budgets such a large amount of space for game updates can trigger this problem in certain situations. Adding Gallery content can corrupt saves but shouldn't delete them, and it should never delete any saves where the content hasn't been placed.
However, if you two use the same Microsoft account, that could be the problem. The workaround would be to change OneDrive settings to stop syncing or to unlink one computer from OneDrive entirely. Both are covered in the thread linked above.