Once she's installed RealVNC, you need a VNC
client on the Mac, not a server, if you're on a Mac and want to control her system.
I use
Chicken of the VNC all the time. It's free and, although a bit slow, very useful. I control a PC and a Mac in my parent's house remotely (from thousands of miles away) with no issues. As mentioned, you just need the IP address, and you'll need to have port 5900* forwarded to her system from her router and, I think, on your router to your system.
* 5900 is the default port. You can use any port you like, as long as RealVNC sets it, CotVNC is set up to use it, and it's forwarded appropriately.
Just bear in mind that both are bandwidth pigs; I'd use a separate phone line if possible if either of you have not a lot of bandwidth.