The selfish mother who changes suddenly is more dangerous. She saw that she is losing her son, his trust and his true affection for her. What she does? She suddenly starts supporting her son's love and agrees to bring Sona as bahu. She feels so good about herself for being so magnanimous ...to herself!
She will go to any extent. Why? She wants her son back. Eternally indebted to her. Again in her hands.
Once Sona becomes wife of her son, what happens? Whenever she sees Sona, she will feel it is a slap on her face. She will think that Sona conquered her son and humiliated her. Her insecurities will start to haunt her once more. Sona will become the wall between her and her son. Unless the wall is demolished, she cant be truly close to her son.