First of all, let us just evaluate the need and advantage of fingerprint scanner in phones.
The security of the phone became a major concern when the phones got smarter. The utility of the phones increased in the recent times making the phone a very crucial, important and sensitive information holding device. Thus the security issued were addressed by using passwords. The typing of passwords were made easy with the use of pass code (PIN) and pattern.
However speed was the demand of the time. The generation does not wanted to enter the pin/pattern/password each time to unlock their phone. So the companies tried and found out new methods for unlocking phones. Among them the most popular one was fingerprint security. Almost all manufactures incorporated the tech into their devices. The feature allowed instant unlock of the device at a tap. Fingerprint security, up to extend, achieved the goal of quick unlocking security. Still, the risk of unlocking the device when you are asleep is there.
Now we will get into the issue. You could see the option for managing fingerprint security under Settings-> Screen Lock and Password-> Manage Finger Print. The phone provides the option to store five fingerprints at the same time, any of which can unlock the device. However, you will have to set up a base security (either Password/Pattern/PIN) which can also be used to unlock the device as a backup. So if you plan to give your phone to your sister, you can choose any of the options. Either let her know the screen lock password or add her finger print to the available slots.
I prefer the latter as it would be more convenient for her and more secure for you. More secure for you in the sense that screen lock in the phone can do many things which a fingerprint cannot. For example if your sister knew the password, she could ad more fingers or delete the existing ones. However she could not do it if she is given the fingerprint option.
Hope you get everything right. Regards..