YOU SAID:

My mother‘s mother is the niece of my father‘/s son in law

INTO JAPANESE

私の母親の母親は私の父親の義理の息子の姪です

BACK INTO ENGLISH

My mother's mother is the mother-in-law of my father-in-law

INTO JAPANESE

私の母の母親は私の義父の義母です

BACK INTO ENGLISH

My mother's mother is my father-in-law's mother-in-law

INTO JAPANESE

私の母親の母親は私の義父の義母

BACK INTO ENGLISH

My mother's mother is my father-in-law's mother-in-law