There are no equivalent in English for this term, but there are many ways to translate it. You need to know the relationship of this person in 子夜 to someone else and use that relationship to translate it. It can be translated as someone's wife, daughter, or cousin, etc. Or even better, if you can figure out her name, just translate this term with her name.

It is a polysemous word, which can also mean great grand parents' daughter.

It might be easier to find some translations within the same genre, such as 围城 (Fortress Besieged) by Qian Zhongshu. I'm sure you will come across something similar. Sometimes, it can easily be Mrs. XXX (surname) – I think it is the address/title such as 姑太太/姑奶奶, which often refers to a person's paternal aunt who is married, as opposed to 姑娘/小姐.