The answer to that , the root cause of it is the one and only Shanti Devi.!
Lets look at this . Shanti Devi scraped her own hands and threathen Rani that she would put the blame of that on her. She threatened to commit suicide and also put the blame on Rani who has become mad in Amit's love. Of course she knows she will have public sympathy on her side and also she has sufficient evidence to make her look guilty as hell.
Now lets go back 10-20 years when she would have been a much stonger woman. Imagine what she did and threaten Koushilya with in the same manner. Of course Raguwa will believe his mother over his wife especially if there is a pinch of truth to what she is saying.
As Sarla said " you speak a lie long enough and loud enough others start to believe it."
To top it off, Koushilya dose not even defend herself she just takes the blame quitely. So Preeti dose the same. She is jumpy as a cat. I don't doubt that as children this woman probably put the fear of God into them if they disobeyed. Preeti has her mother's nature so she takes it.
This is bad because in the future should she be married, he will most likely end up as some ones door mat because of her fear of speaking up!!