What is the ruling on the husband's sister sitting in front of him in revealing clothes, and his family discussing very private matters in front of me?

A woman should not show her mahrams (unmarriageable kin) more than what is commonly exposed of her body, such as the head, neck, hands, and feet. What your husband's sister does, uncovering what is unlawful to uncover in front of her mahrams, is a reprehensible act. You must denounce this and explain Allah's ruling to her and to your husband, and that it is not permissible for him to look at what is unlawful from his sister. He must command her to observe the obligatory hijab (covering). His mother or sister should not mention matters related to intercourse and the like in front of him without necessity, for modesty is part of faith and a characteristic of Islam.