You and your sister have to activate your FaceTime accounts by going to settings > FaceTime,click on FaceTime and set up the account.Make sure you each know the other's email address.
Making a FaceTime call
To make a FaceTime call, choose someone from your contacts or favorites, or list of recent calls.
When you open FaceTime, you may be prompted to sign in using your Apple ID, or to create a new account.
Call a contact: Tap Contacts, choose a name, then tap the phone number or email address the person uses for FaceTime. FaceTime must be turned on in Settings > FaceTime.
Restart a recent call
Tap Recents, then choose the call from the list.
Add a favorite
Tap Favorites, then tap the plus sign, choose a contact and select the phone number or email address that the person uses for FaceTime.
Call a favorite
Tap Favorites, then tap a name in the list.
Add a contact
Tap Contacts, then tap the plus sign.
For a contact outside your region, be sure to enter the complete number, including country code and area code.
Set options for FaceTime
Go to Settings > FaceTime.
While on a FaceTime call
While talking to someone using FaceTime, you can switch cameras, change the orientation of iPad, mute your microphone, move your picture-in-picture display, open another app, and finally, end your call.
Switch between the front and back cameras
Tap the camera icon.
Change the orientation of iPad
Rotate iPad. The image your friend sees changes to match.
To avoid unwanted orientation changes as you move, lock the iPad orientation. See Viewing in portrait or landscape.
Mute the call
Tap the microphone icon. Your friend can still see you, and you can still see and hear your friend.
Move your picture-in-picture display
Drag the inset window to any corner.
Use another app during a call
Press the Home button , then tap an app icon. You can still talk with your friend, but you can’t see each other. To return to the call, tap the green bar at the top of the screen.
End the call
Tap the phone,end icon.
