1. You are in a complex situation,
2. Her in laws are not taking any interest in the matter. It is not a crime as per law and they can not be prosecuted for it,
3. What is required is expeditious completion of the enquiry which is still at the PI (Preliminary Inquiry) stage,
4. You can file a W.P. petition before the High Court against police inaction to get an order directing the police to act expeditiously,
5. Unless, her husband is implicated in the case, you can not get the custody of your sister's children.