302, 328 Viscera does not show poison

I am ajay kumar. I got married on jan 2017. On second day of marriage my wife tolds to my family she was in pressure to get marry to me by his family. We called her parent but they did't picked our calls. After too many calls her uncle came to my hous and she left with him by saying she is mentally sick. After 2-3 months her family start putting pressure on my family to give half of my property to her otherwise they will put dowry and women harrasment case on us. One day on 10 Jul they came to my house and took my father with them. After 2-3 they came with my father to home. My father got fainted. They told us its due to weather as it was too hot in july in north india. After some time he start vomitting . We admitted him in hospital where on next day he got died. We have put FIR in name of 8 people including my wife. But viscera report does not shows poison in the report. Now police have put FR stating that since there is no poison. So we have no proof now. What could be done now.