Family issues

My family consists of four members. My parents, my brother and me. My brother got married two years before and they had one girl child. My brothers wife is very sensitive and not honest towards our family. Initially she was good but later onwards she getting influenced by her mother. Her mother is not at all a honest person she used to behave in one way in our presence but in the absence of our presence she used to degrade our families self respect. Now she started asking my mother to send her daughter for one month. This was refused by my mother because my mother got to know that she( wife of my brother) was influencing by her mother but after their request for three to four times my mother and my brother send her to her mothers home. Then they are saying that they will send her daughter whenever they want. Then my brother got angry because intially they said only one month but now they are changing their word's. I just want to know how to deal with situation because if we try to argue with their family then we may have a chance to get blamed by others as the other side was a women. So kindly tell us how to deal with this situation.