You cannot connect Skype to Facetime. However, you sister can install the Skype app, from the app store, on her iPhone. Then, assuming she has a Skype name/account, you can have Skype-to-Skype video calls between your PC and her iPhone.
Typical Apple baloney. Skype is open and will work on Apple devices without a problem, but Apple closes their FaceTime application for Apple only. That's the reason I won't buy Apple products and probably t he reason that Android is eating their lunch in mobile phones now.