Hi,
As a start, please ask your sister to make sure she is using the latest MAC version (1.7). If this doesn't help, please ask your sister to open a support request as it seems like the problem has to do with her computer. It's a good idea to include as much information as possible such as if she has a built-in camera or other type of webcam, her operating system, details about her video experience, etc.
Thanks for using ooVoo and I hope this issue can be resolved quickly,
Marvin