A man wants to take a second wife – would the second wife be sinning?
Question

                My cousin and I love one another; he came to propose marriage to me but my mother refused. So he married someone else and had two daughters, but three years later he came back and wants to marry me according to the law of Allaah and His Messenger, and he wants to divorce his wife because of the problems and difficulties between them that have nothing to do with me. I love him but I am afraid of doing wrong to his wife; I do not want to bear any sin if I marry him. May Allaah reward you with good.
Praise be to Allah.
    There is nothing to stop you marrying him, whether he 
    divorces his wife or not. Your marriage to him would not be counted as 
    wrongdoing towards his first wife, because plural marriage is something that 
    is regarded as commendable according to sharee’ah for the one who is able to 
    treat his wives fairly. With regard to the problems between him and his 
    first wife, and his thinking of leaving her, you have nothing to do with 
    that, and you will not be sinning, so long as you do not ask him to divorce 
    her or encourage him to divorce her in any way. 

    But if he does not divorce her and he wants to marry you, 
    then he must treat you both fairly. If he fears that he will not be able to 
    treat you fairly then it is not permissible for him to engage in a plural 
    marriage, because Allaah says (interpretation of the meaning): 

    “… then marry (other) women of your choice, two or three, 
    or four; but if you fear that you shall not be able to deal justly (with 
    them), then only one or (slaves) that your right hands possess”

    [al-Nisa’ 4:3].