Dear Mr. Medic, This is what I would do. Having said that, I know sweet FA about etiquette; and my way of doing things is probably all wrong. Are you close to your brother? If so, I would first ring him and simply tell him the truth (as you have in your, rather sweet, OP e.g.). Your brother may make an exception and come up with an alternative arrangement for you. If not, I would then formally respond to the RSVP. If you are not particularly close to your brother, I would simply respond to the RSVP, with perhaps a little explanation.