My twin sister can unlock my iPhone with her face

See this support document - About Face ID advanced technology - Apple Support

“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.”

You should have known that it is common sense, your twin looks exactly or very similiar like you, right? Apple has even stated that a twin can be able to unlock your phone with Face ID. I suggest you disable Face ID and use a passcode or get an iPhone with Touch ID.

It's very common for young family members to be able to use a siblings face ID and unlock it.

Same here

Disable Face ID and use a passcode. Unfortunately, as shown by other responses, it can happen, especially with twins.