Q: Are wifi or cellular needed for face time with iphone?

For Factime, you need to have a device that supports it, if using something less than iOS 6.0 you need wifi. You can use cellular if you have iOS 6 or above and your carrier supports it.

Only the iPhone 4S, iPhone 5 and iPad 3rd and 4th Gen can use FaceTime over cellular.

True, but I guess I made an assumption when he said his sister could Facetime from her home. My mistake. OP, if your sister does not have a device that supports Facetime over cellular and your mother does not have Internet access then that would be a restriction.