My sister was playing Pokemon Y, doing some matches in the battle institute. One of her opponents was a Wailren. Wailren used Rest, and then Sleep Talk. Sleep Talk chose Sheer Cold, and it hit. Was was happening?
Nothing on Bulbapedia says that Sleep Talk interferes with the chosen move's accuracy, so I think it's safe to say that moves chosen by Sleep Talk do not always hit. Your sister was the unlucky recipient of Battle Institute RNG. While OHKO moves have low accuracy, just because they hit doesn't mean there have been modifications to their accuracy. It's the luck of the draw.