"She feels her own self in another form has occupied her mind and controlling her activities" and "She also felt a guilt feeling that mentally she had sex with her younger son and she will become pregnant mentally" .In my opinion it is a psychotic phenomenon going on, it can be a part of the bipolar disorder
but not necessarily. She needs to be treated with antipsychotics and mood stabilizers first, then antidepressant can be considered afterwards.
If she is not taking medicine there are 2 options, you can consult your doctor as doses are required:
1. You can get a depot antipsychotic injection that works for 2-4 weeks.
2. You can give her medicine in water, it comes in liquid form as well.