What is the psychology that mother-in-law always speaks ill of her daughter-in-law?

Answer: Jealous, jealous of the good relationship between son and daughter-in-law.

Extension:

Jealousy refers to people's psychological state of indifference, contempt, exclusion and hostility towards people who should be United in order to compete for certain rights and interests. Therefore, once left unchecked, it may lead to jealousy and make people feel sad. In serious cases, people will naturally feel disgusted.

Jealousy: it is the expression of bad relationship between people and the expression of feelings between people. An emotional and psychological state caused by resentment and awareness of the benefits enjoyed by others and the desire to keep them for yourself.

Reference source: Baidu Encyclopedia-Jealousy