RSo I'm trying to underline language that sounds peculiar or unusual semantically from the text below.
Would I be right in saying that 'the neice of inspiration' is peculiar, because being 'the niece of inspiration' doesn't literally mean anything. It's semantically 'peculiar', a metaphor in other words.
Metaphors are not necessarily semantically peculiar. I'd say that "the niece of inspiration" is peculiar, but "the mother of invention" is not. This is because the latter is used widely, so it's not peculiar.
Is there anything else in the text which would be considered semantically peculiar or is what I said above wrong and am I going about this the wrong way?
I think you have the right general idea. Why don't you try first by underlying what you think is peculiar or unusual?
'Even if it keeps you up all night, wash down the walls and scrub down the floor of your study before composing a syllable. Clean the place as if the Pope were on his way. Spotlessness is the niece of inspiration.
The more you clean, the more brilliant your writing will be, so do not hesitate to take to the open fields to scour the undersides of rocks or swab, in the dark forest, upper branches, nests full of eggs.
When you find your way back home and stow the sponges and brushes under the sink, you will behold in the light of dawn the immaculate altar of your desk, a clean surface in the middle of a clean world.
From a small vase, sparkling blue, lift a yellow pencil, the sharpest of the bouquet, and cover pages with tiny sentences like long rows of devoted ants that followed you in from the woods.'