Is it permissible for a man to conceal from his current wife and family the matter of his returning to his former wife, and is that considered an injustice?

There is no harm for a man in concealing his second marriage from his wife and mother, nor is there any sin upon him in that, and it does not involve injustice. However, he must be just between the two wives in terms of lodging, dwelling, and other rights.

Disobeying the mother in this marriage is not considered an act of disobedience (uquq); for the forbidden uquq is when the child causes significant harm to the parents عرفًا (customarily), or disobeys them in matters where there is fear for the child's life or a limb. This marriage has a legitimate religious benefit, which is the gathering of the daughters with their father and their receiving his care and upbringing.