What is the meaning of 一草一木 here?
一草一木 is an idiom, originating from Book of the later Han:

《後漢書・應劭傳》：「春一草枯則為災，秋一木華亦為異。」
In spring, a single blade of grass that withers heralds disaster; in autumn, a single tree that flourishes is unnatural.
That is, every small detail or single thing out of place suggests unnatural occurrences.

This was later shortened to 一草一木, meaning every small/single thing.

我是一無所有，吃穿用度，一草一木，皆是和他們家的姑娘一樣...
I have nothing at all; everything I use - what I'm eating or wearing, every single thing - are the same as the girls of that family...