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. We ask Allaah to exalt his mention as well as that of his family and all his companions.
If this man married your sister via a valid marriage contract fulfilling all the conditions of a valid marriage, most important of which is the consent of the guardian of the woman, then she becomes his wife and remains as his wife if he has not divorced her. Not consummating the marriage throughout this period does not affect the marriage contract.
Instead of thinking of divorce, it is better to discuss the matter with the husband and think about the possible ways to join them. However, if it is not possible and there is no way but to divorce, then your sister has the right to ask an Islamic judge to cancel her marriage which is Islamically known as Khul’.
Allaah Knows best.