If this is all on one ticket you''ll more than likely get the rest of your ticket canceled. Have you contacted the airline to find out what it would cost to change your ticket to the itinerary you want? It might be worth the extra cost to avoid hassles.

You might luck out with being too ill to fly to Rio and have the airline agree that you'll make your own way to Rio and they'll keep your return intact but make sure they actually do that last part or you'll end up screwed.