How can a sister-in-law say thank you when her parents are sick and her sister-in-law takes care of her?
You can say: Sister-in-law has worked hard, and mom and dad are ill and in poor health. You take care of them. You are not afraid of being dirty and tired, regardless of the day. I am very grateful as a sister, and I thank you. I will definitely learn from Sister-in-law in the future, go home often and help her. It is a blessing for our family to have you at home.