No setup, no installation, no router configuration, completely free*. Download the executable and run it, have your sister click start and read you her ID number. You download the exe file, run it, go into the "operator" tab, enter her ID, and click connect. She will be prompted to allow your connection, and when she does, you will be connected.
I've used Ammyy many, many times to help out family and friends; it's way simpler than setting up VNC or RDP over an internet connection. Gets you out of all that sticky port forwarding or VPN connections, and you won't have to walk your sister through configuring any clients on her end. All she has to do is download the program, run it, click a button and read you a number.
Also, performance is surprisingly good; I personally get speeds close to using RDP over an internet connection, but it's going to depend somewhat on where you are in relation to Bangkok.
Edit: *by completely free, I meant for personal use. There appear to also be restrictions of 15 hours per month on the free license (they state "possible" restirictions); I've never used it that frequently, so I've never come across that. It's not open source or freeware, but for one-off connections (such as helping out your sister), you really can't do better. Sorry if my original post was misleading, I hope this clarifies.