Their father treats them and their mother harshly so they have decided to cut him off
Question

                A father’s harshness. My father married my mother who is a simple woman who knows nothing more than respect and obedience. She did not go out of the house or go to school or college, whilst he loves women and pursues them. He never paid any attention to my mother’s feelings and he always tells her that she is naïve. My brother and sister and I grew up with no one raising us except our simple mother. Now my father always argues with us and insults us in an embarrassing fashion and uses bad words. My concern is the pleasure of Allaah, as we have taken a stance against him and we do not speak to him or serve him. Is Allaah angry with us? Also, he does not pray; I always urge him to pray, but with no success.
Praise be to Allah.
    Firstly: 

    The fact that your father does not pray and loves women and 
    pursues them are obvious evils, the most serious of which is his not 
    praying, because not praying is kufr that puts a person beyond the pale of 
    Islam, according to the more correct of the two scholarly opinions. There is 
    a great deal of evidence for that, including the words of the Prophet  
    (peace and blessings of Allaah be upon him): “Between a man and shirk and 
    kufr there stands his giving up prayer.” Narrated by Muslim (82). Hence you 
    have to advise your father and do not despair of his repenting and being 
    guided. You should look for various means of advising him, such as giving 
    him a tape about the rulings on the one who does not pray, and seeking the 
    help of family and relatives who could advise him, and so on. 

    The fact that he has mistreated you and is still doing so 
    means that you must have compassion towards him in your hearts, for if he 
    were to die and meet his Lord with these deeds, he would meet Him with great 
    sin and disobedience. 

    Hence you and your siblings and family should look again at 
    your relationship with your father and your attitude towards him. Allaah, 
    may He be glorified and exalted, has commanded us to treat our parents well 
    and to honour them even if they are kuffaar and call us to kufr. 

    Allaah says (interpretation of the meaning): 

    “But if they (both) strive with you to make you join in 
    worship with Me others that of which you have no knowledge, then obey them 
    not; but behave with them in the world kindly”

    [Luqmaan 31:15]

    Ibraaheem (peace be upon him) debated with his mushrik father 
    in a polite manner, as Allaah tells us (interpretation of the meaning): 

    “And mention in the Book (the Qur’aan) Ibraaheem 
    (Abraham). Verily, he was a man of truth, a Prophet.

    42. When he said to his father: ‘O my father! Why do you 
    worship that which hears not, sees not and cannot avail you in anything?

    43. ‘O my father! Verily, there has come to me of the 
    knowledge that which came not unto you. So follow me, I will guide you to 
    the Straight Path.

    44. ‘O my father! Worship not Shaytaan (Satan). Verily, 
    Shaytaan (Satan) has been a rebel against the Most Gracious (Allaah).

    45. “O my father! Verily, I fear lest a torment from the 
    Most Gracious (Allaah) should overtake you, so that you become a companion 
    of Shaytaan (Satan) (in the Hell‑fire).’

    46. He (the father) said: ‘Do you reject my gods, O 
    Ibraaheem (Abraham)? If you stop not (this), I will indeed stone you. So get 
    away from me safely (before I punish you).’

    47. Ibraaheem (Abraham) said: ‘Peace be on you! I will ask 
    forgiveness of my Lord for you. Verily, He is unto me Ever Most Gracious’”

    [Maryam 19:41-47]

    Look at the etiquette of this Prophet  (peace and 
    blessings of Allaah be upon him), and how he addressed his mushrik father 
    who threatened him. This offers an eloquent lesson for those who are tested 
    with such fathers. 

    Secondly: 

    From what we have mentioned it is clear that the father’s 
    rights to honour and good treatment are not waived if he falls shorts and 
    mistreats his family. Rather everyone should fulfil the duties that he owes. 
    So even if he falls short, you should not fall short. Moreover, turning away 
    from him, forsaking him and not serving him will make him even worse, and 
    undoubtedly you do not want that. Many of those who sinned and then repented 
    have spoken of the effect of people’s forsaking them, and said how that made 
    things worse and made them do stupid things that they would not have done if 
    they had felt that they had children who loved them and respected them even 
    though they were harsh and behaved badly. 

    No matter how bad a person is, he does not forget kind 
    treatment, even if outwardly he appears to be ignoring it. Hence the most 
    beneficial way of dealing with your father is to draw close to him no matter 
    how far away he seems, and to be merciful towards him no matter how harsh he 
    is, whilst also praying that he be guided aright. 

    Rest assured that your kind treatment will not be wasted 
    before Allaah, and it will bear fruit for you in this world and in the 
    Hereafter, by Allaah’s leave. 

    We ask Allaah to guide your father and set his affairs and 
    all your affairs straight. 

    And Allaah knows best.