Why not try the Carolina Herrera look? I always thought her crisp white shirt with a maxi skirt was understated elegance, easy and its covered up. You could glam it up with clutch, heels,belt, accessories etc.
I think if you were invited then they appreciate your presence and welcome you and they know how you dress already and they haven't considered it a problem. A knee length dress with a shawl should work fine or just be yourself. Muslim weddings especially arab wedding are fun, you will be surprised!