[threeseed]

A twin could walk into any bank with their sibling's passport and do anything they wanted.

Banks are all about risk management not necessarily avoiding and and all risks.

[esolyt]

But a twin wouldn't be able to unlock my iPhone 7 with their finerprint.

[threeseed]

You just disable FaceID in that scenario and go back to using a passcode.

But again if you can't trust your twin well then your phone is the last thing I would be worried about.