Problem with child custody after parents divorce

My sister in law's husband cheated on her when their daughter was about 2 year old. Her husband got back to her realizing his mistake and wanted to work things out, however she could not forgive him and wanted a divorce. Due to family pressure they were just living together showcasing as if they were trying to work things out but in reality they were not even talking to each other. The daughter is now 7 years old. My sister in law over the last few years found a partner, she divorced her husband without getting the family involved and even got married to her new partner. The issue over the child's custody is a big problem now. Per the divorce terms the mother has the custody of the child and father only has visitation rights. The child is now uncomfortable to live with her mother and her new partner. Mother is not willing to give her ex husband child's custody, she is ok to leave the child with child's grandparents. Should grandparents take custody of child in this case or should we just let the child get accustomed to her mather's new relationship and let her be with her mother. What would be best for the child..?