I am a identical twin and I would expect my twin brother to be able to unlock my phone. Personally I like knowing this, so if something where to happen to me, he could access my phone.
There are always going for be folks out there who can get by any security system. Doesn’t matter if it’s fingerprint sensors and face recognition.
I have faith the Apple is aware of this and with all its previous devices, they will have an update to strengthen the feature.
Until that time I don’t think there are more then two of us out there, so I am not concerned.