How can we help reunite my sister and her husband after a year of living separately?

My sister who got married 13 years back and she has one son 12 years and one daughter 11 years old. From the day she got married she is not happy with her husband. No bonding between their relationships. When he got married he didn’t had proper job. So my dad decided to keep them in our home and we built one separate floor for their family to live. Days passed, my dad joined him in government job. They were happy for few months. They lived separately in rented house thrice but weren’t happy. But still he started torturing her that he wants to live in a separate house and I want to take care of my mother and all that. Her mother does not want his son to be with her because she did not come to her son’s marriage itself. This problem happened before marriage. Now the situation became very worse that my sister planned for “khula”. So many times we dealt to bond them. It has been a year they are living separately; she is in our home and also her children. Please guide us to join them.