Warning against marrying one whose chastity is in doubt
Question

                I recently got a mail from one of my relatives.The mail was sent to me by mistake.In that she had written about her affair with a boy.She stated that she had crossed all her limits.Now the problem is that no one knows about it and i dont intend telling it to anyone but my grandmother had selected that girl for my brother.I can't say these things to my parents and neither to my brother.My father listens to eveything that his mother says so it is very likely that he will agree to this match.If my grand mother comes to know about this girl also she isnt going to tell my parents anything about it as the girl is her sisters daughter.So what should i do? please advise..
Praise be to Allah.You should note first of all that the letter which came to you by mistake may have 
    been fabricated against this girl. 
Secondly, if you are sure that this letter is genuine, then one of the two 
    following scenarios must apply: 
1 – That this girl has started to change and reform, and she has ended her haraam 
    relationships. In this case there is nothing wrong with your brother marrying her. You should cover up for her, because the Prophet  (peace 
    and blessings of Allaah be upon him) said: “The Muslim is the brother of his fellow Muslim; he does not wrong him or let him down. Whoever meets 
    his brother’s needs, Allaah will meet his needs, and whoever relieves a Muslim of some worldly distress, Allaah will relieve him of some of the 
    distress of the Day of Resurrection. Whoever covers a Muslim (conceals his sin), Allaah will cover him (conceal his sin) on the Day of 
    Resurrection.” 
Narrated by al-Bukhaari (2310), Muslim (2580). 
See also question no. 4882, 
    26810. 
The Prophet  (peace and blessings of Allaah be upon him) has told us that 
    repenting from sin erases it. He  (peace and blessings of Allaah be upon him) said: 
“The one who repents from sin is like one who did not sin.” Narrated by Ibn 
    Maajah, classed as hasan by al-Albaani in Saheeh Ibn Maajah, 3427. 
See also questions no. 13990, 
    14289, 27176. 
2 – If this woman’s religious commitment and character are not satisfactory, or 
    there is no guarantee that she will not go back to haraam ways, then you should advise your brother not to marry her, because the Prophet  
    (peace and blessings of Allaah be upon him) said: “Religion is sincerity (naseehah).” We said: “To whom?” He said: “To Allaah, to His Book, to His 
    Messenger and to the leaders of the Muslims and their common folk.” Narrated by Muslim, 55, from the hadeeth of al-Tameem al-Daari (may Allaah be 
    pleased with him). 
That should not be done by accusing her or exposing her, rather it should be done 
    by trying to divert your brother from marrying her, such as by sending him a letter or telling him directly that marrying this girl will do him no 
    good. If that does not put him off marrying her, then you should tell him that you know something that means that he has to forget about marrying 
    her, and other things that will make him give up the idea of marrying her, even if you are forced to tell him what happened with this letter, 
    without slandering her. 
And Allaah knows best.