I think Mannat wants revenge for the baby she lost because of her brother's callous treatment in his hospital. Maybe she can never become a mother again. As she had dreamt of motherhood it had got a psychologically unhinged. Since her intention was to snatch the thing/person he holds dearest, she could have created the misunderstanding between Om and Sia and in a fit of anger, Om may have murdered his wife. Since his wife disappeared under mysterious circumstances, the finger of doubt pointed to him. It may have been around this time he came across a wounded and unconscious Tara whom he had nursed in coma for 5 years to let the real world think he is the devoted husband.
I would have thought that a simpler way to get rid of her brother is to let the police know what goes on in his hospital. Why go to great lengths to frame her brother with the murder of his 'wife'? Unless Mannat is not aware of Om's other side. That looks unlikely, or is it?