I don't get the complaints about battery life either.
I get home with hight 60's after work.
and then I played about 6 games of ingenuous, sometimes even a game of hearthstone, or three and we know that app is TERRIBLE for battery life.
I check twitter when I smoke after my game of ingenuous, and Taptalk and Telegram are going all day.
screen on time by 5pm is around 2.5 hours and then I still have 60% left.
I think the probelm your sister is having is she:
a) uses the phone while it is on charge.
Leave the thing alone for 2 hours damit, you will survive! it will make the battery last longer!
b) unplugs the phone before it gets to 100%
this means the reserve of the battery is not full and that eats her battery life.
c) she has some sort of battery drainign app installed, something like the old faebook app. using data and battery even when the phone is locked and in her pocket.
I am using faecbook as an example, I deleted my facebook account in 2011, when I realised how bad the android app was.
also yes comparing to the Z3 is silly. Sony did an amazing job with that phone's battery, but I believe that if you use the G4 100% correctly it will out last the Z3. simply because it is newer. and the battery is bigger.