2274796 said:It would appear the easier option.
I have to say that a double duvet is warmer on a single bed as it doesn't slide off to one side every 30 minutes and leave half your body out in the cold. I've with Night Train's Mum on this - if I see a guest single bed with a single duvet, I see a bad night's sleep coming up. Those who think otherwise must sleep like corpses.
It can be made worse as a guest - for example, if your sister has put every major domestic appliance on night-time timers for cheap rate electricity and they all come on at 2am with a noise like a bomber squadron flying underwater.