I hear your concerns. I regret to inform you that currently, to ensure that only the subscriber has access to the account information, users are required to log in their My StarHub App (MSA) to view the details of Data Usage.
I suggest monitoring your usage via your own account monthly via your handset, or a 3rd party app.
Alternatively, you can only switch on auto login for My StarHub App, and request that your sister log in once for you. Perhaps you can also consider checking with your sister from time to time for the data usage as well.