Thanks for all your comments. If this was in England and not in KSA I wouldn't think twice.

I actually called an agent to make the booking and explained very carefully that only my wife and brother in law will be there. He disappeared for half an hour and then confirmed that the booking had been made. But then the email had my name as guest.

FWIW my wife and her brother have the same last name on their passports. My wife kept her original last name after marriage.