She has got enough problems in her life already with a wayward alcoholic husband, constant battles with her mother and now a mystery illness, that she wouldn't even dream of sabotaging her brother's life and future.
Plus its her brother alone who seems to give her something positive to look forward to in life. Which is why her whole focus now is him and his future.
I strongly suspect the bro-in-law and Prithvi to be in this together. Prithvi seems to be getting fishy after each episode. And I'm sure that KK will also be actively involved.
Its surely not the mom or sister. But the mom might unintentionally cause some harm as she seems to blindly trust KK.
KK is likely to be behind it. No surprisingly, Aditya does not like him. Let's also keep in mind Mehek Ahuja, whose threatening call may have drawn Aditya out in the open at night, making him an easy target for assassination.
i thought bro in law first...bt when mehak said someones whos CLOSE TO HIM,i suspect Prithvi... no way his sis n mom toh bilkul nahi...so only Prithvi left