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.