This is practically the same as a business setting up a file server. I have a readyNAS at home but haven't researched sharing it. At work, I have a RAID fileserver that I access from home, at work, all over (which gives me some experience with your question).
In the bigger picture, here are the steps in no particular order:
- have a static IP from the ISP where the server lives (for your home internet service)
- set up port routing in your firewall (usually the wireless+router box)
- find out which ports you intend to serve, send them to the internal IP address of your NAS
- set the DHCP server in your router to start at .100 or somethign
- set your NAS to a static IP address within your LAN to a value outside the range of the DHCP server
For my work machine, it is running Ubuntu, a samba server, and openssh-server. At the workplace, I route port 22 (ssh) to this server. Within work, windows computers can connect to the IP address of the server, authenticate, and share files. This is handled within the samba.cnf configuration and there is a LOT of info on the internet on how to do it. I have no idea how you would securely do windows file sharing over the WAN, though ssh tunneling would work if you can figure that out.
For across the internet, I use SSH, SCP, and SSHFS to connect computers and share / edit files and I use id_rsa.pub and private keys for authentication. All of these use the ssh protocol for the connection. I do not know the explicit steps required to get Windows to share files, but I'm sure it isn't too complicated with googling. I know I use "putty" in windows to SSH in to ubuntu computers.
So, to get you on to letting a windows computer from outside your network share folders via windows sharing:
- find out the port(s) to forward
- find out the encryption and authentication schema available
- test locally on your LAN
- then test from work, etc
- publish a how-to for your relatives
- answer the phone when they call for help ;)
This isn't a "how-to" but should have enough info that you can get working on your goal. :)