Why is my iPhone 13 unlocking with both my Face ID and my brother's?

Review the information provided here. I don't know the context of how you are determining this, such as you being in the vicinity of the camera while watching or if you brother knows your passcode and claims that he can unlock it with Face ID. Using Face ID continually updates the appearance after a failed attempt and the passcode is entered. This allows for changes in your appearance, but will also learn the features from another person who has used the Passcode and may be more notable in siblings will many facial features would be more similar.

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. 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. You can also use Face ID without enabling Face ID with a mask.

About Face ID advanced technology - Apple Support

It's not unusual for close blood relatives to have facial structures that are close enough to trigger Face ID, even though it seems like you don't look that much alike.

If this creates a problem for you, disable faceid.

I’ve experienced this too—both my sister and I have had it happen. Even though we have completely different eye and hair colors, our facial structures are similar enough that Face ID sometimes gets confused and recognizes us as the same person. If your sibling frequently uses your device, Face ID may start adapting to their features as well, which can cause recognition issues. In cases like that, the best solution is to reset Face ID and set it up again using only your face. That usually clears up any confusion and restores accurate recognition.

If that still doesn't work, just don't use face ID altogether and stick to passcode.