My first and only heartbreak came at the age of 66, until then I had no clue how horrible it is. And it lasts a long time after a relationship of so many years. Mine was 29 years. Your sister will never be the same again but… he may be back when he finds the grass is not greener on the other side of the fence. Time will tell. He’s smug now because he knows he can get her back in a second. If she can go no contact with him and keep away he will get curious and maybe even jealous and she will heal faster.