@FreshFarmSale, if your sister wishes to use her device to ring up sales for your Square account, all she needs to do is install the Square Register app and then log in using your account information. You don't need to add her to your account...she'll just use your login.
And if you ever have a need in the future to give someone the ability to make sales on your account, but you don't want them to have your login and full access to all the settings, you can add them as an "Employee" using the Employee Management feature.
Square allows you to track sales by device and by employee, FYI.
