Face ID recognizing sister's face on my iPhone

Does she know your Passcode? With a failed attempt to recognize your face, the data will be updated to note changes when the Passcode has been entered.

• 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.
• The probability that a random person in the population could look at your iPhone or iPad Pro and unlock it using Face ID is less than 1 in 1,000,000 with a single enrolled appearance whether or not you're wearing a mask

About Face ID advanced technology - Apple Support

If she knows your passcode then reset your Face ID and consider not sharing your Passcode. The iPhone is only a single user device not meant to be used by others.

Please review the information already provided including the link where you will find additional info. The solution is also the same.

An additional point that may be relevant from the same Support Article is:

• As an additional protection, Face ID allows only five unsuccessful match attempts before a passcode is required. The statistical probability is higher — and further increased if using Face ID with a mask — for twins and siblings that look like you, and among children under the age of 13, because their distinct facial features might not have fully developed. If you're concerned about this, we recommend using a passcode to authenticate. 

Well, you said it was very strange, but that is the normal operation of updating the Face ID data after detection has failed and the correct Passcode has been entered. In some age related cases even without the other party knowing the passcode, it can happen. You did not say if your brother knows your Passcode so I provided the additional information.

My brother can unlock my phone too with Face ID. It’s very strange, because it only works sometimes. My face is the only appearance.

I wasn’t saying it was a problem I was just telling the author it happens to me too