بسم الله الرحمن الرحيم
As it is fardh for a woman to cover all his body in the same way it is also mandatory for her to wear such a clothe which does not expose the size and contours of body. It is mandatory to avoid wearing clothes which are see-through or tight and expose the structure and outline of the body.
The Holy Prophet Muhammad (may peace and blessings of Allah be upon him) said, “Women who are naked even though they are wearing clothes, go astray and make others go astray, and they will not enter the Garden and they will not find its scent.”
Also there is resemblance with westernized and fashionable persons and it promotes immodesty. It is narrated in an Hadith: من تشبه بقوم فهو منهم.“He who imitates any people (in their actions) is considered to be one of them.”
Thus you should stop your sister wearing such dress which is not acceptable as per the Shariah. (Chand Ahem Asari Masail, vol. 1, p. 318)
Allah knows Best!
Darul Uloom Deoband