A paranoid question; can people access my computers documents folders wirelessly?

This is a very paranoid question;
But I'm a writer and don't want certain people stealing my ideas and whatnot -
Can someone wirelessly access my computer's documents and folders? and I don't mean like, if they are sat outside in a car trying to hack my wireless connection. I mean like - can someone access your desktops folders, files and documents over distance? Not just wirelessly but any way?????

What has led me to asking this question is that my sister and I used to use the same laptop and recently she bought a new mac, and the day after she exclaimed 'wow, my new mac has somehow uploaded all my folders from the old laptop without me telling it to'. Does this mean someone with a mac a mile away can access my files? lol

help a paranoid person out please lol :P

thanks