Is it considered disobedience to parents if children cut ties with their mother who abandoned them when they were young and then returned, and now wants to go back to her husband whom they consider to be an unfit person?

The mother was obligated to care for her children and not abandon them or deprive them of her affection and upbringing, even if she remarried. Her marriage invalidates her right to custody, but not her right to be honored, maintained, and treated kindly. Nevertheless, it is not permissible for the children to sever ties with her. Rather, it is obligatory for them to honor and treat her kindly, no matter how badly she behaves, because her misconduct does not negate their duty to honor her. It is not permissible for the children to encourage their mother to divorce unless there is a legitimate reason, such as the husband's فسق (open sinfulness), provided that the divorce does not lead to a greater harm.