The relationship between son-in-law and parents-in-law should be Weng Xu's relationship. My father-in-law is the father of my son-in-law's wife, and my son-in-law is my daughter's husband. Although the father-in-law and son-in-law are not related by blood, they belong to the closest relationship and are also in-laws.

The relationship between father-in-law and son-in-law is called Weng Xu, and the relationship between son-in-law and mother-in-law is called "gu Xu". Generally speaking, men and women do not address each other side by side, otherwise it is derogatory, so there is no fixed word for the relationship between mother-in-law and daughter-in-law.