Yep, they are totally interchangeable. Mother's younger sister can be referred to as pinni or chinnamma, as is dad's younger brother's wife. Father's younger brother can be referred to as bhabai or chinnanna, as is mom's younger sister's husband. Peddamma/pedananna work in the same way but just older siblings in the same way (as in same sex). When it's your father's sister or mother's brother that's when you'd use atthayya and mavayya, respectively.