Will a person be sinful if he sees his sister immodestly dressed, and she does not listen to advice, and what should he do about the abundance of sins he witnesses from his family?

The questioner must advise her sister and clarify the limits of a woman's awrah (intimate parts) in front of her mahrams, along with the prohibition of looking at what is between the navel and the knee of her body if it becomes visible. If she fulfills her duty of advising, then there is no blame upon her.