We ask Allah, may He be exalted, to recompense you for your calamity, and to relieve you of your pain, and decree reward for you. And we ask Him, may He be exalted, to guide your father, for he has acted very badly indeed by deliberately neglecting his family whom Allah, may He be exalted, commanded him to look after and take care of, especially since those whom he neglected of his family are those who are weak. Your father has also been unjust towards your mother by not giving her her rights of maintenance and not being fair between her and his other wife. And he has been unjust in terms of giving, as he has given to his children from his other wife but not to his children from his first wife. All these things that your father has done are clearly sins and neglect of the obligations that Allah, may He be exalted, has enjoined upon him, so he deserves the warning unless he repents to his Lord, gives up this wrongdoing, establishes equal treatment of his two families, and sets straight what he has done wrong. If he does that, he will find that his Lord will accept his repentance and show mercy.
Despite all the things that your father has done, his right to kind treatment and obedience in that which is right and proper is still guaranteed, according to the shari texts. If Allah, may He be exalted, has mentioned the rights of the mushrik father - and even the one who calls his children to associate others with their Lord, may He be glorified and exalted - to kind treatment and good companionship, then the one who is less than him in terms of evildoing is more entitled to that kind treatment and good companionship. Allah, may He be exalted, 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, and follow the path of him who turns to Me in repentance and in obedience. Then to Me will be your return, and I shall tell you what you used to do
Although the father deserves to be warned (of divine punishment) for his sin and neglect of shari duties, disobedient children and those who do not treat their parents kindly are also warned about their actions; it is not permissible to repay mistreatment with mistreatment or wrongdoing with wrongdoing.
Although children are not to be blamed for feelings of resentment in the heart towards the father who commits sin or is a disbeliever, that does not contradict the duty to treat him kindly and obey him in that which is right and proper. But you have to hold your tongue and refrain from speaking badly to him, and also refrain from mistreating him in practical terms.
As the issue was caused by your father's actions and you have gone through so much pain and hardship, we advise you to seek reward with Allah for what you have gone through. And we advise you to offer duaa' for your father, praying that he be guided and enabled to repent and set things right, because he is in the greatest need of the mercy and forgiveness of Allah, may He be exalted.
See also the answer to question no. 148924.
And Allah knows best.
Reproduced from Islam QA