like as Nu_du wrote, Vandana looks suspicious. I watched some scenes and even felt the same now. She does look fishy.
Then, Sid is always found near to the incidents. why?
Divya also keeps on repeating that Nivi is unwell, she seeks attention and does everything as she is not well.
As another member pointed out, Viren drugged Nivi also before and that too just for a whole night, so that she can get up in the morning. On one hand, we can say that he was trying to help, or another hand, we can also say, that he has knowledge of drugs and also that time by rendering his helping hand, he was trying to impress Nivedita.
CC and JW are always in the suspicion list. Nandu or Lalji kaka also look suspicious.
But I think YD is not in this scheme. CC just utilises YD's hot temper to their favour and YD being a rigid and egoistic lady always lands up siding with the wrong people.