Look at it this way, your not being paid, the brief is for relaxed candids and there is no pressure to deliver the "goods"
So relax! Take one camera, one lens.
Enjoy the day and don't worry if your getting the shots you think you should be getting and instead, shoot what you see as a guest.
Too many people will tell you to make sure your kitted out with spare bodies, batteries, lenses, spare lenses, memory cards, flash guns, spare flash guns and yes, if it were a paid gig, you were expected to deliver its essential to ensure every possibility and angle is covered.
But back to your brief, you have been told that its a low key affair so for you to portray this then surely the best way to approach is... Well, low key?
I attended a cousins wedding last month and my sister was asked to cover the wedding, i naturally offered to shoot some impromptu stuff. I took along my main camera, flash, triggers, bag, batteries, cards ect and as we were traveling to the reception i thought what is wrong with me? I've been invited to enjoy the day, not worry about carrying around a bag, camera, framing shots and ducking and diving around my family for good light.
So i ditched all the gear, picked up my rangefinder and three rolls of tri x and enjoyed myself and the resulting images show, people were more relaxed and hardly noticed i was taking their picture half the time!
Up to you but sometimes, less is more.