hello @Amanda660
Ask if the brother can be added to the account to cover all bases.
It's probably the most sensible and civilized less stressful way for all parties to address this matter at such late notice.
How would you feel if you for instance were in the same situation and the couple with an infant?
It's better to have happy Guests than a cancellation, isn't it?
I had a situation a while ago whereby a family member booked accommodation for another one who due to their personal situation was unable to use a computer and it worked out fine.
Focus on the positive, it's family.
It's great to see the UK opening up again
All the best from across the miles and oceans in New Zealand