niece
姪 centers on a female relative one generation down: a niece, specifically a brother's daughter.
姪 combines 女 (woman) with 至, which likely contributes the sound. The character specifically denotes a niece.
A woman 女 and 至 (which looks like a person arriving) together suggest a female relative who has come into the family: a niece.
For テツ, imagine your niece playing with a toy Tetsujin (鉄人) robot: Tetsu -> テツ, and she's the little girl 女 in the family.