wives of one's brothers, sister-in-law
妯 centers on a specific female family relationship: the wives of one's brothers, or a sister-in-law by marriage.
妯 combines 女 (woman) with 由, which likely contributes a phonetic element. The exact historical development is uncertain.
The woman radical 女 beside 由 suggests a female relative connected through a shared origin or reason (由). Picture a woman who becomes your sister-in-law through marriage, linking two families.
For チュウ, imagine choosing (チュウ sounds like 'choose') a sister-in-law from among your brothers' wives: choose -> チュウ, and the woman 女 is the one you pick as family.