Who lost her husband to someone else
Who lost her son when she decided to take her husband back
Her husband cheated on her
Her son started showering his love and faith on his mother in law
Hence she is possessive of her first son. She wants his love and his trust back. She will go to any length for that.
She must have pained to see that her son has been betrayed again and hence tried her way to help him by killing her DIL.
I am not justifying her. Had Adi supported his mom, Anjana would have never become the psycho that we get to see today. But what to do, Adi is equally flawed.
Her second son has always been neutral. So she has no fear of losing him.