His sister is studying in a mixed university; should he spend on her?
Question

                My younger sister lives in Bangladesh and is living in Dhaka so that she can do her university degree. Is it okay first of all that she lives there with her sister and brother-in-law? I know sometimes that my sister is alone at home with the brother-in-law when her older sister if out. Is this okay? And secondly, is it okay for my younger sister to being going to a co-ed university in Dhaka? She is studying business and there are men and women studying together. Sometimes they have assignments which are given to them and they end up being in groups that have non-mahram men and women working together. Is this islamically acceptable? There is another university in Dhaka which is single-sex teaching. Should I send her there instead? And lastly, is it okay for me to finance my sisters education if she does not want to change universities and stays at the co-educational institute? Should I stop sending money for her education if the environment is not halal?.
Praise be to Allah.
      The sister’s husband is a non-mahram for a woman, like all 
      other “strangers”, so it is not permissible for her to show any part of 
      her body in front of him and it is not permissible for him to look at her 
      or shake hands with her or be alone with her. What you have mentioned 
      about her sometimes being alone with him is something that is undoubtedly 
      haraam, and it is a cause of temptation and evil, because the Prophet 
      (blessings and peace of Allah be upon him) said: “Do not be alone with a 
      woman unless there is a mahram with her.” Narrated by al-Bukhaari (1862) 
      and Muslim (1241). And he said: “No man is alone with a woman but the 
      third one present is the Shaytaan.” Narrated by al-Tirmidhi (1171); 
      classed as saheeh by al-Albaani in Saheeh al-Tirmidhi. 

    Al-Nawawi (may Allah have mercy on him) narrated in Sharh 
    Muslim (14/153) that there was consensus among the scholars that it is 
    haraam for a man to be alone with a woman who is a non-mahram. This was also 
    mentioned by al-Haafiz in al-Fath (4/77). 

    See also question no.
    13261. 

    Secondly: 

    It is haraam to study in co-ed schools and universities 
    because the haraam things and evils that result from that are obvious to 
    everyone, such as looking, being alone with a member of the opposite sex, 
    speaking softly, infatuation and things that are even worse than that. This 
    has been discussed previously in detail; see the answer to question no.
    45883 and
    8827. 

    Thirdly: 

    What you have to do is advise your sister and explain to her 
    that it is haraam for her to stay in her sister’s house if that will result 
    in her being alone (with her sister’s husband) or taking lightly the matter 
    of looking or hijab. You should also tell her that it is haraam for her to 
    study in a co-ed university and you should try to make her study in an 
    environment where there is no mixing, even if that means studying at home or 
    distance learning. It is not permissible for you to help her in this haraam 
    study with any of your wealth, because Allah says (interpretation of the 
    meaning):

    “Help you one another in Al‑Birr and At‑Taqwa (virtue, 
    righteousness and piety); but do not help one another in sin and 
    transgression. And fear Allaah. Verily, Allaah is Severe in punishment”

    [al-Maa’idah 5:2]. 

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

    And Allah knows best.