i think what happened is arnav's dad was responsible for kushi's parents death as he accidently ran over the car into kushis parents car. when kushi's parents die garima takes kushi as her own daughter. arnav's dad feels guilty for the accident and he tries to help garima and kushi by giving money and support as they are poor. maybe someone mad mouthed about arnav's dad and garima or someone misjudged them and they told arnav's mum about it/ maybe arnav's mum saw arnav's dad going to garima's house. then she misunderstood arnav's dad and could not trust him and in pain she committed suicide. he loved his wife so he committed suicide to forget the pain.