It does look like it's beyond saving. Did it start to wilt like that when you were not watering it or only after your sister started over-watering it? I have had a lavender (Lavandula dentata) as a houseplant for a number of years. It lives on a sunny windowsill in winter and outdoors in its pot in summer (after being gradually acclimatized to the outdoors).
One thing I have found, and it surprised me, is that it wilts very quickly on a sunny day on the windowsill, even in winter, and needs watering far more often than I would have expected. If yours started drooping before your sister started watering it, then that would indicate insufficient water rather than too much. If it had gone beyond what is appropriately called the "permanent wilting point" any amount of water that your sister gave it would not have brought it back.