daughter-in-law
媳 centers on a woman who enters a family through marriage: a daughter-in-law.
媳 combines 女 (woman) with 息, which likely contributes the sound and the idea of a child or offspring, together suggesting a woman who becomes a child of the family by marriage.
A woman 女 beside 息 (which can mean child or breath) suggests a woman who becomes a child of the family: a daughter-in-law.
For セキ, imagine a daughter-in-law sitting at a family gathering, feeling a bit like a 'seki' (席, seat) has been prepared for her: seki -> セキ.