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

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.