What's the difference between elder sister-in-law and younger sister-in-law?

Dad's sisters are generally called aunts, aunts and nuns. Dad's sister is called Sister, and Dad's sister is called Sister-in-law. Of course, the unified title is aunt.

Dad's sister is usually called aunt. Dad's sister is called Sister, and Dad's sister is called Sister-in-law. Of course, the unified title is Auntie, and there are some dialects in some places. For example, some areas in eastern Henan and Sichuan will be called "Auntie", while some areas in Ningbo will be called "Momo" and "Auntie". But in fact, what you become is what your aunt means. Your elder sister-in-law is just to distinguish two relatives with the same relationship.