Should he interfere to prevent the marriage of a girl to a person with whose brother she had a relationship?
Question

                Is it allowed to marry with a girl who had relation with the elder brother of the boy(Allah knows the extent of relation but they were like girl friend and boy friend). The boy doesnt know the relation of this girl with his elder brother and no one else in both families but some friends of his elder brother does know that. Now the both families has decided their marriage, so tell us is it allowed or they shouldnt marry ?.
Praise be to Allah.
      There is nothing wrong with the girl marrying this suitor, 
      subject to  two conditions: 

    1.That she has repented to Allah 
    from the haraam relationship, and if it reached the point of zina, it is not 
    valid to marry a woman who committed zina until after she repents.

    See the answer the question no.
    85335. 

    2.That there is no danger of a 
    resumption of her relationship with his brother. This marriage may make it 
    easy for the relationship to resume and continue, especially in households 
    where no attention is paid to preventing free mixing. That would cause a 
    great deal of trouble, as is obvious.

    Based on that, if it is known that she is still in this 
    relationship or there is the fear that it may resume, based on what is seen 
    of circumstantial evidence, then in this case the one who has knowledge of 
    the situation should interfere in order to prevent the marriage, so as to 
    denounce evil and protect the honour of the Muslim. So he should advise the 
    suitor to forget about this woman in a way that will not lead to conflict 
    and division between the two brothers; rather he should speak in general 
    terms, such as saying; “She is not suitable for you,” or “she is not good 
    for you,” and so on. 

    If it is known that the relationship has ended and there is 
    no fear of it being resumed, then praise be to Allah, and there is nothing 
    wrong with this marriage, as stated above. 

    And Allah knows best.