younger sister
姨 centers on a female relative, specifically a younger sister. The meaning is narrow and familial.
姨 combines 女 (woman) with 夷, which likely contributes the sound. The character was used historically for a younger sister or a wife's sister.
A woman 女 beside 夷: picture a younger sister standing next to her older sibling, the 夷 shape suggesting a person of similar build.
For テイ, imagine a younger sister taking tea with her older sibling: tea -> テイ, and the woman radical 女 reinforces the family scene.