@Noel8, I agree with @Marit-Anne0. Third-party bookings are not condoned by AirBnB. I would decline the request (if indeed it was a request and not an inquiry), and inform the person that her brother needs to do the booking for himself and the grandmother.
Don't worry if you lose this particular potential guest. There will soon be others. 🙂
Make sure that whoever books your listing at any time has ample verification ID and at least a couple of good reviews from other hosts. And be alert to any inconsistency in the initial request. Her addressing you by a wrong name implies she didn't read your listing carefully if even at all.
I would also suggest you do the following:
- require a Security Deposit;
- change your cancellation policy from flexible to moderate or even strict;
- create a new set of House Rules that are more comprehensive and very firm; guests need to be made explicitly aware of the type of behaviour that you will expect from them.
Meanwhile, be sure to read the thread 'Community Help Guides' that's near the top of the first page in the General Hosting section of this forum. It's full of information that's essential to any - especially new - host.