If she leaves the country during pendency of her own cases against your brother, she may stand losing all the cases due to non-prosecution of the same. However she cannot be legally stopped from leaving the country if there is no criminal case pending against her. In the divorce case she is just a respondent, if she decides not to respond to the case , the case can be decided exparte thus benefiting your brother, hence ask him not to bother about her leaving the country. However if your brother is concerned about the child to be taken away by her to abroad, he may file a child custody case and seek for an injunction from she leaving abroad with the child or to hand over the child to him if she intends to travel abroad. consult your lawyer and proceed as per his further advise.