What if someone in your family lends you their iPhone 5s for the day?

How will they access it with the new fingerprint unlocking method?

My brother doesn't exactly need a phone but if he needs to go somewhere alone - my dad lends him his own phone… Does that mean my brother will be unable to use the phone?

No you can de activate the pass code or just use a number one and I'm sure you can change the fingerprint on settings

The IP5s allows the owner to set other users fingerprints to be recognised and to set appropriate restrictions upon how they may use the phone. I don't yet know how many people can be authorised, but it's unlikely to be very many.