Is complaining about the immodest dress of one's husband's sister, or about the mother-in-law's treatment, considered backbiting, envy, or jealousy?

It is obligatory to advise the girl and explain the obligation of the Islamic hijab (definition) and the prohibition and danger of tabarruj (wanton display), unless advising her would lead to significant harm or corruption. The advice should be given gently and wisely, utilizing available resources such as tapes and books, or by directing her to sermons and lessons. As for speaking with the brother's wife about the flaws of the girl and her mother, it is permissible if it serves the purpose of advice and does not constitute forbidden backbiting; otherwise, it is not permissible. It is important to ensure sincerity of intention and purity of purpose, as jealousy for the Shari'ah may sometimes coincide with urges of the self.