To erase all data and return the tablet to factory settings you can perform a hard/factory reset. This will allow you to erase all your sisters data and restore the device to factory settings. You can then set up the tablet the way you like with your accounts and apps and so on. Since there is currently a lock screen in place you will need to boot into recovery mode to perform a hard/factory reset. To do a factory reset on the tablet follow these steps:
Resetting the Tablet
Power the device down completely, and then hold both the volume up button and the power button until the RCA logo appears on the screen. When the screen changes to the Android bot on his back, press and hold the power button and then press volume up once more. At this point, the Android System Recovery screen appears, with a list of options. Use the volume up and down buttons to navigate the menu and make selections accordingly.
To reset the device, scroll to "wipe data/factory reset" and press the power button. Confirm the selection by scrolling down through several "no" entries until the word "yes" is visible. Highlight "yes" and press the power button. The process takes between 1 and 10 minutes, but once it finishes, it powers up again and shows the "Welcome" screen. This is a sign of a successful reset. Try to make sure the battery is around 50% or higher before performing the reset.