Your brother and his wife have filed a divorce petition by mutual consent . According to the provision of law, the consent of both the parties to the divorce petition should continue to exist till the disposal of the petition by granting decree of divorce by the court. Otherwise, the petition will be dismissed by the court. However, in this case, your brother's wife appears to be non-cooperative may be because she is likely to extract more money from your brother under the pretext of withdrawal of her consent or because her lover may not be in a position to accept her or because of some other unknown issues. You just find out the reasons for her such behaviour and act accordingly.
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