YOU SAID:

my mother is your step-mother who is my grandmother

INTO JAPANESE

私の母は私の祖母であるあなたの継母です

BACK INTO ENGLISH

My mother is your stepmother who is my grandmother.

INTO JAPANESE

私の母は私の祖母であるあなたの継母です。

BACK INTO ENGLISH

My mother is your stepmother, my grandmother.

INTO JAPANESE

私の母はあなたの継母、私の祖母です。

BACK INTO ENGLISH

My mother is your stepmother, my grandmother.