Presumably it means the same thing as it does for hens.
Sorry, but "nest-ripe" is not a very common English term. It's very hard to talk about it in the abstract. If you could give the hen-centric explanation, and provide the context in which you saw this term being applied to young women, we could probably help you understand the metaphor better.
I should avoid quoting, for it's from a text that hasn't been published yet (and some sentences may be different in the final version), but I can try to retell it: the author is comparing two young women. One is still unmarried and promising to be beautiful, but her beauty is still in bud. The other one is described as of a more mature and refined beauty, nest-ripe and willowy. Just that.