大姐:big elder sister, the eldest sister
二姐:second elder sister, the second sister
三姐:third elder sister, the third sister
大妹:big younger sister
姐夫:elder sister's husband, brother-in law
妹夫:younger sister's husband, brother-in law
丈母娘:wife'mother, mother-in law
婆婆:husband's mother, mother-in law
看了这些, 能否找到一点规律?