wife of elder brother
姒 refers specifically to the wife of one's elder brother, a kinship term within a traditional family structure.
姒 combines 女 (woman) with 以, which likely contributes a phonetic element. The character is a specific kinship designation.
A woman 女 beside 以: imagine a woman introduced as 'the one who comes after' (以 can suggest sequence), fitting the role of the elder brother's wife in the family order.
For シ, picture the elder brother's wife gently saying 'shh' to calm the children: 'shh' -> シ, and she is the 姒 of the family.