wife of father's younger brother
嬸 refers specifically to the wife of one's father's younger brother, a precise kinship term for an aunt by marriage on the paternal side.
嬸 combines the woman radical 女 with 審, which likely provides the sound. The character was created to represent a specific female relative by marriage.
A woman 女 who examines 審 family matters closely—like the wife of your father's younger brother, who often has a watchful role in the extended family.
For シン, imagine the aunt (嬸) shushing everyone at a family gathering: 'Shin!' she whispers, and the sound シン helps you remember her title.
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