Newly married girl behaving differently.

My cousin newly got arranged married to a girl. Before marriage was settled the girl agreed not to go for a job as He is well to do in business. Only son, so he is staying with his parents. His parents performed marriage as if it is their daughter not daughter in law. Soon After marriage with in a week, the girl even convinced his husband and in laws to get a job. From that time onwards I.e. Within a week of marriage, she is disrespectfully bahaving with in laws that top only mother in law. When her mother in law is trying to counsel her how to behave with others in a respectful manner, she is still disrespectfully treating her. In laws waited for one month, till that time they tried of their own to counsel the girl, but she never tried to change. Recently she went to her parents house of her own intention stating that her mother in law is not treating her properly. Later when both the sides had discussion, the girls family members only accepted that the girl herself mistake that she doesn't know how to behave that She is taking every thing in a easy going manner and shouting at her in law for no reason. In this one month she has time and again threatened to leave house, not willing to eat at home even after returning home from job at night, talking disrespectfully with mother in law especially, time and again threatening that she will go to court and divorce, told the boy that she has agreed for marriage for the sake of her parents even though she doesn't like marriage and after return from mothers house she has even thrown away the mangalsutra. Her parents say that she has lot of anger and behaves the same way at their home before marriage also. What ever the point was raised and counseled by mother in law the same points were already being done at her mother house before marriage and was counseled on same grounds by her mother also. She was also advised by her mother not to be like that at her in laws house and behave properly. But she is refusing to accept this even after being counseled by her own family members. She continues to behave the same even after this also which her parents also are worried about now. She is not at all trying understand and change her attitude even after all these. Top of this she still feels that she has not done any mistake. When trying to counsel by her parents she is turning her own statements that she has not done these things and that she is only being disrespected by the boy's family for which there is no proof. What if the girl commits some thing or tries to put blame on the boy's family? Boy side still expect her to change herself as counseled by her parents which she is not at all willing by continuing to do the things as before. Being soft natured the boy and his parents are vexed with her behavior. More so the boy is totally upset with the girl disrespectfully treating his mother. It appears as if she is concerned about her own self and not about others even husband. Request you to Kindly advice.