If his sister goes out wearing a hijab and pants, what should he do?
Question

                Does a hijabi sister who wears wide-leg pants come under the heading of women who are clothed yet naked? Is her guardian regarded as a cuckold (duyooth)? What should her brother do if she rejects his advice, if he is her guardian?
Praise be to Allah.
    Firstly:

    Hijab, as 
    prescribed in Islam, has certain conditions and characteristics which have 
    been explained in the answer to question no. 
    6991. 

    It is haraam for a woman to appear in 
    front of non-mahram men wearing pants, because they do not cover her ‘awrah 
    in the prescribed manner; rather they show her charms, which is known to be 
    haraam. 

    Shaykh Ibn ‘Uthaymeen (may Allah have 
    mercy on him) said: 

    I think that the Muslims should not follow 
    these fashions and kinds of clothes that come to us from here and there. 
    Many of them are not in accordance with proper Islamic dress which involves 
    covering the woman completely, like the short or very tight or thin clothes. 
    That includes pants, because they show the size of a woman’s legs, as well 
    as her stomach, waist, breasts, etc. Wearing it comes under the description 
    given in the saheeh hadeeth: “there are two types of the people of Hell whom 
    I have not seen: people with whips like the tails of cattle, with which they 
    strike the people, and women who are clothed yet naked, walking with an 
    enticing gait, with something on their heads that looks like the humps of 
    camels, leaning to one side. They will never enter Paradise or even smell 
    its fragrance, although its fragrance can be detected from such and such a 
    distance.” 

    This hadeeth was narrated by Muslim, 
    2128. 

    End quote from Majmoo’ Fataawa al-Shaykh 
    Ibn ‘Uthaymeen.  

    Secondly: 

    The prohibition on wearing pants is lifted 
    if they are worn under another garment, but if they are worn under a blouse, 
    even if it comes down to the knees, that is not permissible, because the 
    reason for the prohibition is still there. 

    Shaykh ‘Abd al-Razzaaq ‘Afeefi (may Allaah 
    have mercy on him) said: If a woman wears pants and puts over them a wide 
    garment then she will not be resembling men, so long as she wears them 
    underneath her garments. End quote from Fataawa al-Shaykh ‘Abd al-Razzaaq 
    ‘Afeefi, p. 573. 

    Thirdly: 

    The woman’s guardian should encourage her 
    to do that which is good, and tell her not to do that which is evil; he 
    should make her adhere to hijab and prevent her from wearing clothes that 
    constitute wanton display (tabarruj). 

    Allah, may He be exalted, says 
    (interpretation of the meaning):

    “O you who believe! Ward off from 
    yourselves and your families a Fire (Hell) whose fuel is men and stones, 
    over which are (appointed) angels stern (and) severe, who disobey not, (from 
    executing) the Commands they receive from Allah, but do that which they are 
    commanded”

    [at-Tahreem 66:6]. 

    And the Prophet (blessings and peace of 
    Allah be upon him) said:

    “Each of you is a shepherd and each of you 
    is responsible for his flock. The ruler of the people is a shepherd and is 
    responsible for his flock. A man is the shepherd of his household and is 
    responsible for his flock. A woman is the shepherd of her husband’s house 
    and children and is responsible for her flock

    Narrated by al-Bukhaari (893) and Muslim 
    (1829) 

    The brother should and advise his sister 
    and remind her of Allah, may He be exalted, and of what He has enjoined upon 
    her of observing hijab and covering herself. That will be a means of raising 
    her status in this world and the hereafter. He should also remind her of the 
    warnings against making a wanton display and showing one’s adornments in 
    front of non-mahram men. If she responds, then praise be to Allah, but if 
    she persists and he is able to prevent her from going out except when 
    necessary, then he should do that, so as to denounce the wrong deed and 
    reduce the evil, and so as to ward off the negative consequences that may 
    result from her making a wanton display. But if he fears that that will lead 
    to a greater evil, he should stick to advising her and verbally objecting to 
    what she is doing. 

    Shaykh Ibn ‘Uthaymeen (may Allah have 
    mercy on him) was asked: 

    When the women in our village go out to 
    the fields or they go out of their houses, they wear their best clothes and 
    put on perfume, but at home with their husbands we find that they do not 
    care about that. If we try to advise them, they do not listen. 

    He (may Allah have mercy on him) replied:

    For a woman to go out to the marketplace 
    wearing perfume and adorned in beautiful and charming clothes is something 
    haraam; it is not permissible because of the temptation that may result from 
    that. Her guardian should prevent her from going out in this manner, whether 
    he is her husband, her father or her brother, because this is a wanton 
    display (tabarruj) and Allah, may He be exalted, says (interpretation of the 
    meaning): “and do not display yourselves like that of the times of 
    ignorance” [al-Ahzaab 33:33]. My advice to these women who go out to the 
    marketplace wearing these clothes and this perfume is that they should fear 
    Allah concerning themselves and their community, for if they go out to the 
    marketplace wearing these clothes, the people will be tempted by them and 
    they will become the talk of the town. So they must fear Allah, may He be 
    glorified and exalted, and not go out wearing perfume or make-up

    end quote from 
    Fataawa Noor ‘ala ad-Darb

    He (may Allah have mercy on him) also 
    said: The woman’s guardian must prevent her from wearing any kind of haraam 
    clothes or going out wearing her adornment or perfume, because he is her 
    guardian, therefore he is responsible for her on the Day of Resurrection, on 
    the Day when a person shall not avail another, nor will intercession be 
    accepted from him, nor will compensation be taken from him, nor will they be 
    helped. (cf. 2:48). May Allah help us all to do that which He loves and 
    which pleases Him.

    End quote from 
    Daleel at-Taalibah al-Mu’minah

    We do not say that the case of this hijabi 
    is exactly like that of one who makes a wanton display; rather we say that 
    if her hijab does not meet the conditions of hijab stipulated in Islam, then 
    she should fear Allah, may He be exalted, and observe proper, correct hijab 
    in submission and obedience to her Lord, may He be glorified and exalted. 

    May Allah help us all to do that which He 
    loves and which pleases Him. 

    And Allah knows best.