Yes! My parents love to change their mind all of the time so this has happened a lot. So far we have always been accommodated. I just ask very nicely at the podium and am always prepared for a big no. So I would just be honest with your brother, hey, we will try and theres a good chance it will be OK but there's also a chance they could say no. You may want to ask about an hour earlier than your reservation just so your brother can make other plans if need be.