"I thought she was you!" He feels horrible for protesting with that, especially in Elena's presence and he has to stop himself from casting a glance her way to see how she responded to the comment. "I woke up here, Katherine. They moved me. Take it up with the hamsters for sticking me in your sister's bed."

Stefan purposely throws 'sister' into that, as if to remind her, in case she's forgotten in her outrage, that this is unlocked and certain appearances need to be kept up.