All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
First of all, we ask Allaah to facilitate your affairs, relieve your sorrow and mend your conditions. We recommend you to supplicate Allaah, as it is with supplications that worries are removed and difficulties are solved; and especially to supplicate Allaah with some specific supplications like the one narrated by Abu Bakrah that the Prophet said: “Allaahumma rahmataka arjo fala takilni ila nafsi tarfata 'ayn, wa aslih li sha'ni kullahu, la ilaaha illa ant.” (O Allaah, it is Your mercy I desire, so do not entrust my affairs to myself even for the blink of an eye and put right all of my affairs. None has the right to be worshipped except You.) [Ahmad and Abu Daawood]
On the other hand, the husband should be wise in case of dispute between his family and his wife, and he should not wrong one party at the expense of the other, but he should fulfill the rights of each. Therefore, if your sister's husband abuses her, and she was harmed by that, then she has the right to seek divorce in order to repel harm from herself. But she should not hasten to do this; rather, wise and rational people from her family and her husband's family should sit together and try to reconcile. Allaah says (what means): {And if a woman fears from her husband contempt or evasion, there is no sin upon them if they make terms of settlement between them - and settlement is best. And present in [human] souls is stinginess. But if you do good and fear Allaah - then indeed Allaah is ever, with what you do, Acquainted.} [Quran 4:128]
It is bad that neither your sister's husband nor his family came to visit and see his wife and her baby after she gave birth.
On the other hand, your parents should not order you to divorce your wife as a way of vexing her family. If your wife is pious, then you should keep her, be kind to her and not divorce her. Obedience to parents in this case is not obedience in what is permissible and reasonable. For more benefit, please refer to Fatwa 84808.
Keeping your wife is not considered undutifulness to your parents. However, you should try to please them [after keeping your wife] in order to avoid them being angry with you.
If your wife is forsaking you and does not talk to you as you mentioned, then she is wrong. Also, it is not permissible for her to obey her parents if they order her to do so, as there should be no obedience to a created being in something that involves disobedience to the Creator. For more information, please refer to Fatwa 124826.
In conclusion, we emphasize what we have mentioned above that the help of wise arbitrators should be sought in order to solve these problems.
Allaah Knows best.