Is the wife considered sinful for seeking divorce due to the ill-treatment of her husband's family and her husband's siding with them, and for refusing to see her child after birth to pressure him, then regretting it after her divorce and being deprived of her child, and is she entitled to request custody of her child now?

It is permissible for a woman to seek divorce from her husband due to harm, and there is no sin upon her for doing so. Likewise, it is not permissible for her to relinquish custody of her son out of defiance towards his father, nor is it permissible for the father to prevent her from visiting her son; rather, she has a greater right to his custody. She has the right to demand her custody, and if the father refuses, she may file a lawsuit to establish her right. It is preferable to attempt reconciliation and mutual agreement before resorting to the courts.