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 is His slave and Messenger.
If your sister is harmed by staying with her husband, then she may ask for divorce because of the harm. However, if she wants to stay with her husband and be patient with him, then you should not encourage her to seek divorce from him.
In any case, you should not hasten for divorce, rather, it is more appropriate to be patient and seek a solution for reconciliation because reconciliation is a good thing. This husband is not obliged to bring his wife to reside with him where he resides unless she had conditioned this on him or that her guardian conditioned this on him before the marriage and he agreed to this condition. The Prophet said: “The conditions that must be most fulfilled are those by which you made the private parts (of your wives) lawful.” [Al-Bukhari and Muslim]
However, the mere promise of his family about this without his knowledge does not make him obliged to fulfill it.
Therefore, we advise both parties to respect each other and have mutual understanding about the matter with wisdom, and appoint rational people from both families. Finally, if it is possible to reach an understanding, then this is what is required, and if not, then separation between them should be in kindness.
Allaah Knows best.