Is a husband considered a cuckold if he divorces his wife and denies the lineage of their eldest son after he grows up and graduates from university, given that there are doubts about his lineage and that he is his nephew's son?

One must fear Allah and abandon the satanic whispers that corrupt the family. Accusing one's wife of adultery and disowning a child's lineage cannot be based on doubts, but rather on certainty or a strong presumption akin to certainty. It is not permissible to disown a child's lineage after remaining silent about its attribution for a long period, as disowning has conditions, including not delaying. Some jurists stipulated disowning immediately upon pregnancy, while others allowed a slight delay.

One must be wary of the warning of the Messenger of Allah, peace and blessings be upon him, against whoever disowns his child, or whoever introduces someone who is not of their kin to a people. The lack of resemblance between you and your child, or between him and his siblings, is not proof of immorality. The marital bed takes precedence over resemblance, and lineages must be preserved with caution.

As for the wife residing in the family home: If there is a separate place that ensures her privacy and prevents her from mixing with non-mahram men, and the wife consents, then there is no harm. However, if there is unlawful mixing, it is not permissible, and the situation must be changed, and the wife must be isolated from the husband's non-mahram relatives. For the husband's male relatives (hamu) are death, due to the opportunities for suspicion they present.