Someone else can unlock my Face ID

First, reset your Face ID, by turning it off, then back on. If you have ever allowed your cousin to unlock your phone with the Passcode while they are looking at it, then the Face ID data is being updated to reflect any appearance changes. That is how it is supposed to work as people's looks do change. Don't give your Passcode to anyone else.

Sure. The odds of any random individual being able to unlock your device are about 1 million to 1. But, those odds are different for family, twins, and children. And, if you hand your unlocked device to someone else, and they log in using a passcode or you log in for them, their face is learned.

Learn more at About Face ID advanced technology - Apple Support