elder sister, maidservant
姐 centers on a female figure in a familial or domestic role: an elder sister or a maidservant. Both meanings share the idea of a woman in a close, supportive position.
姐 combines 女 (woman) with 且, which likely contributes the sound. The exact historical development of 且 in this character is uncertain, but the structure suggests a phono-semantic compound related to a female role.
A woman 女 beside 且, which looks like a shelf or a helper. Picture an elder sister or maidservant standing by, ready to assist.
For ソ, imagine the elder sister sewing a button: sew -> ソ, and her hands work like a maidservant's.