Eventually this superiority makes them extremely possessive about their son. It makes it difficult for them to share their son with anyone. But obviously it's the societal norm to get their son married, so they are left with no other option.
Now as they are uncomfortable with the sharing part, their mind subconsciously starts searching for negative points in their daughter-in-law and many times in her family as well. This gives them a certain power over the other person and also it is an attempt to keep their superior position intact. When a person blames someone or points out negative things in someone that gives them a feeling of superiority. Hence they treat their daughter-in-law and daughters differently. The height of double standards is many mother-in-laws suddenly change their behavior and be all good with their daughter-in-law if she gives birth to a boy.
Sad reality of our society which hasn't changed till date.