Can she refuse her husband’s request for intercourse if she knows that he will miss Fajr prayer?
Question

                I know that a woman is cursed by all the angels of the paradise if she refuses an intimate relation when her husband orders her. But when the act sexual drive to a major sin or not, she can refuse? When for example she is afraid that her husband does not cleanse himself before the prayer of fajr, or that he drags this state of impurity after an hour fixed by a prayer, to see even some in case he works?If she makes love with him but that she knows that he goes do a sin, makes she a sin also?What do you advise to make in that case there?.
Praise be to Allah.Falling short in performing 
    the obligatory prayers and delaying them until the time for them is over is 
    a major sin, and the one who does that habitually and takes the matter 
    lightly is in grave danger. It will lead to his doom in the Hereafter and to 
    his being a loser before Allaah. 
The wife must also help her 
    husband to obey Allaah and help him to avoid sins as much as possible, with 
    kind words and beautiful exhortation, and by using the means that are 
    prescribed in sharee’ah, not those that are forbidden. 
If you can agree with your 
    husband upon a time after which you will both be able to offer the prayer on 
    time and regularly, then that is best. 
Otherwise, you do not have 
    to do anything other than advising him, and helping him to pray by dealing 
    with him kindly and gently. If he responds, then praise be to Allaah, 
    otherwise he is the one who will bear the sin, not you. It is not 
    permissible for you to refuse to share his bed if you know that he is 
    falling short. Every accountable person must bear the consequences of his 
    actions, and will be asked about what his hands have earned – no one else. 
Moreover, if there is 
    something that keeps the husband from attending prayer in congregation, this 
    is a sin, but it is less serious than his failing to offer the prayer at the 
    proper time. The sin in that case is serious, if the person did it 
    deliberately. 
Whatever the case, we ask 
    Allaah to guide you and accept your deeds, because we can see that you care 
    about your and your husband’s prayer. We ask Him to fill all the Muslims 
    with love for Him and hope in Him and fear of His punishment. 
And Allaah knows best.