Face ID

Try Resetting your Face ID.

It is possible that your device has mistaken your sister to be yourself, as you are siblings, you could look similar. Face ID learns over time, so if you grow a beard/ start wearing glasses for example it will continue to recognise you.

Find out more about Face ID here:

About Face ID advanced technology - Apple Support

Source: https://support.apple.com/en-ie/HT208108

The probability that a random person in the population could look at your iPhone or iPad Pro and unlock it using Face ID is approximately 1 in 1,000,000 with a single enrolled appearance. As an additional protection, Face ID allows only five unsuccessful match attempts before a passcode is required. The statistical probability is different for twins and siblings that look like you and among children under the age of 13, because their distinct facial features may not have fully developed. If you're concerned about this, we recommend using a passcode to authenticate.

Face ID automatically adapts to changes in your appearance, such as wearing cosmetic makeup or growing facial hair. If there is a more significant change in your appearance, like shaving a full beard, Face ID confirms your identity by using your passcode before it updates your face data.

So, change your passcode, reset the Face ID setup and do it again and next time your sister wants to use your iPhone, unlock it first before giving it to her instead of telling her what’s your passcode.

I echo iW00 in suggesting you change your passcode, and then reset / re-register FaceID while in a room by yourself.

Plus …

THEN see if little-sis can unlock it.

After that, NEVER EVER share a passcode.

The passcode not only “opens” the device but is used as a “seed” in the device’s internal encryption schemes as well.

Since you have private info on the device (as most of us do) you could also take this opportunity to “lengthen” your passcode as well.