en_tn/pro/30/23.md

a hated woman when she marries

That is, people rightly hated her before she married; once she marries, she will be worse than she was before she married. This can be translated in active form. Alternate translation: "a woman whom good people have hated when she marries" or "an outcast woman when she marries" (See: rc://en/ta/man/jit/figs-activepassive)

takes the place of her mistress

Possible meanings are 1) the slave girl is able to rule over the wife and children or 2) the man loves the slave girl more than he loves his wife.

mistress

This is the female form of "master," so it refers to the wife of the man of the house.