YOU SAID:
my mom is our mom sister
INTO JAPANESE
私のお母さんは私たちのお母さんの妹です
BACK INTO ENGLISH
My mom is our mom's sister
INTO JAPANESE
私のお母さんは私たちのお母さんの妹です
BACK INTO ENGLISH
My mom is our mom's sister
That didn't even make that much sense in English.