OK, you don't need to establish a VPN.
what you need to do is have your aunts IP address, ask her to go to www.whatismyip.com
to find this out.
you'll also need to log onto her router that connects to her broadband and add port forwarding for port 3389 (RDP port) to the address of her computer, (she should find this out by typing IPConfig at the command prompt).
you also need to right click on my computer select properties and make sure that remote desktop connections are allowed. and you;ll lastly need a username and a password to log on.
now from your machine you should just need to type her IP into the remote desktop client.
this connection will then go from your computer to your router, as it's an external address it'll go out to the internet through various other routers until it hits your aunts router.
from here your aunts router will be set up to know that connections appearing on port 3389 (RDP) need to be sent to a specific computer, so your aunts router will forward the request to your aunts PC.
you don't need a vpn, (though it's recommended for security).
if you use VNC you'll still have to set up port forwarding on the router at her end.