video chat 10.8.2 <-> 10.5.8


I run 10.8.2 with FaceTime and Messages. My sister runs 10.5.8 with iChat. We spent an hour on the phone earlier trying to figure out how to establish a video link between us. From her side, calling my Apple ID email, it responds "user not registered". From my side, calling her Gmail address, it says something like "user not available". Her son, my nephew, calls her Gmail address frequently from his PC - I don't know what program he uses.

Does anyone here have some experience communicating visually between vastly different versions of MacOSX?