All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
First of all, we advise you to adhere to patience, frequently recite Thikr (expressions of remembrance of Allah), and supplicate Allah, The Almighty, to rectify the relationship between your father and husband. Verily, the hearts are in the Hands of Allah, The Exalted, and He is over all things Competent. We hope that adherence to patience, Thikr, and supplication would comfort you and relieve your anxiety and stress as well as any other negative and adverse effects.
Second: If this man married you with a valid marriage contract as per the Sharee‘ah and has not divorced you nor annulled the marriage, then he has slandered you whilst you were his wife.
Merely denying his marriage to you and slandering you by calling your daughter illegitimate does not mean that a divorce or an annulment of the marriage contract took place. The daughter is attributed to him, and if you are pregnant, the fetus is attributed to him as well. He cannot deny his paternity to her or the unborn child except through Li‘aan (mutual oaths of condemnation).
Ibn Qudaamah said in Al-‘Umdah: “A man’s paternity of a woman’s child is not denied except through Li‘aan.” [End of Quote]
In Islam, paternity is confirmed on the basis of the slightest Shubhah (assumption that a marriage is valid while it is not). Ibn Taymiyyah said: “It is agreed upon by all Muslims that if the husband believed the marriage to be valid and thus had sexual intercourse with the wife believing it to be lawful, then a child born to them is attributed to the husband and the mutual rights to inheritance between them are established, according to the agreement of Muslims.” [End of Quote]
Third: It is absolutely impermissible for your husband and father to be in this state of estrangement from one another. Al-Bukhaari and Muslim narrated on the authority of Abu Ayyoob Al-Ansaari, may Allaah be pleased with him, that the Messenger of Allah said: “It is not lawful for a Muslim to forsake his (Muslim) brother beyond three nights; they meet so each turns away from the other, and the better of the two is the one who greets the other first.”
Fourth: The bond of affinity by marriage is sacred and should be honored, being a blessing conferred by Allah, The Exalted, upon His slaves; He Says (what means): {And it is He Who has created from water a human being and made him [a relative by] lineage and marriage. And ever is your Lord competent [concerning creation].} [Quran 25: 54]
Therefore, the in-laws should maintain good relations with one another. We advise you to seek the help of wise people, whose opinion is hoped to be heard and followed, to intervene and reconcile between them. Reconciling between people is one of the best good deeds that draw one closer to Allah, The Exalted. Abu Daawood and At-Tirmithi narrated on the authority of Abu Ad-Dardaa’ that the Prophet said: “Should I tell you what is better in rank than prayer, fasting, and charity?” They (the Companions) said: “Yes (please do).” He, said: “Reconciling between people. Verily, corrupted relations between people is the Haaliqah (lit. shaver; i.e. it uproots faith).”
Fifth: If reconciliation is achieved, then this is the desired outcome, praise be to Allah; otherwise, it is sufficient for you to be on good terms with both your father and husband while continuing to supplicate Allah, The Exalted, and striving to reconcile between them without giving in to despair.
Allah knows best.