If you sister is the original owner, have her contact Apple. Apple can remove the ID if provided the required paperwork to include the original purchase receipt with the serial number on it.
Request additional support for unlocking your device. -
You used to be able to take the device, the original purchase receipt with the serial number, and a photo ID to the nearest Apple Store (if it has reopened) and have them remove the ID. This may have changed, so call the store before going to make sure they can help.
Genius Bar Reservation and Apple Support Options - Apple. -