يا أمي you are calling your mother يا أمت I read it : ya amati I think its origin is- Ama أمة- it means the black slave (the female) and it doesn't mean : Oh my mother .. you can say instead : يا أماه Ya ommah يا أبي : you are calling your father normally يا أبت : It was used in Quran when prophet Ibrahim was talking to his father politely to be kind with him .. and it is used in other places for the same reason : to be polite

يا ابت more love than يا ابي that mean u love u father or u mother more this word u can read it in quran sourat maryam

absolutely it is unusual expression if this expression is exist it will be unfamiliar for me i didn't hear it before although i am native Arabic speaker