You guys forgot the last clause. Here's how I'd write it in my current mood: "She's lying on the floor, naked in the sun, with a boy as delicate as herself sleeping in her arms. And a boy which she really wants to shoot.” ;) In English you don't always have to follow the usage guidelines—if that were the case everybody's writing would sound academic. As you develop into a more complete and confident writer, you find your own rhythm...you develop your own style, sort of like a painter who paints in prose instead of acrylic. That is the beauty of English.

It looks like the above helpers already told you the answer to your question. However, your opening sentence "is there any mistake in this sentence" isn't correct. I'd probably phrase it: "Is there a mistake in the following sentence?" Or "Are there any mistakes in the following sentence?" It depends on how confident you are in the number of mistakes you might have in the sentence. :) Also, simple sentences might be better for you, rather than trying to create an elaborate long sentence.

she's lying on the floor, naked in the sun, with a boy as delicate as herself sleeping in her arms Remove "that"

There is nothing wrong in the sentence as it is a complex sentence and can be read either way and as such the sentence should begin with a captial letter,though it can be written as follows She is lying on the floor, naked in the sun, with a boy who is as delicate as herself sleeping in her arms.

Both Lisa's suggestions are correct, but the first is too long-winded - I would argue