First, thanks for all the responses and help.
I still haven't decided what to use . . . Remote Desktop, Remote Assistance, VNC, CrossLoop, or what. But . . . where I was at one time leaning toward Remote Assistance, and then toward Remote Desktop, now I'm leaning toward CrossLoop.
My main reason for leaning toward CrossLoop is this: The goal of this whole exercise was to troubleshoot a problem on my Sister's computer. Consequently, I don't want to get bogged down and sidetracked troubleshooting the very software that I'm using to get to her machine to troubleshoot in the first place. This might become necessary if I use Remote Desktop or Remote Assistance, because in both cases I would have to tell her how to configure her router (and I barely know myself), or as you suggest, try to tell her how to set up a VPN connection (and that's another thing I'm not very confident doing myself). And too, her router, as I understand it, would give a "disguised" IP address anyway, so I'm not so sure I wouldn't have to get the IP address without the router, whatsmyip.org notwithstanding. Or does that give the "true" IP address . . . is that what you meant by "her external (WAN) IP address"??
So, if I decided to go with Remote Desktop or Remote Assistance I might be inclined to tell her just to unhook the router. But then her problem to troubleshoot may be the router itself (I don't know yet . . . I haven't heard from her).
If she were anything other than computer illiterate, a computer idiot (oops . . . if she knew I called her an idiot, she'd slap me silly) I might try one of those two. But in the case of the router or VPN setup, it would be close to the blind leading the blind, and if it didn't work the first time, I'd likely get sidetracked and end up troubleshooting the setup itself. That would make both her and me frustrated and angry.
Plus there is the issue of TightVNC being "very technical", as you said, so that's out.
So, my tentative conclusion is basically in line with what you said at the end of your post: "Crossloop does sound like a good solution because it does encryption and NAT traversal". And it appears to be the most user friendly, which for my Sister is a big plus. I may end up following Mr. Geeks discussion on it.