People like this don't learn. I got one of my pythons from a friend who convinced her parents that her brother wasn't taking care of it. Her brother also had a black kingsnake that he refused to believe had died, but in all the time I had seen the snake while visiting, I can assure anyone wondering that it was dead the whole time (crispy and gross and very dead). No, I do not know how any snake owner fails to realize their pet is dead. It took me two months to get that python, as he would always ask his sister to get me to pay him for it and offering to throw in the black snake as part of the deal (I am totally serious). Eventually he got annoyed that she kept asking and I brought Chase home with me and got him eating right away.

Maybe your sister and parents will realize she's a bad owner if the gecko dies, or maybe they won't. Maybe they'll notice it died because of her neglect, maybe they just won't care. Sad to say, but this kind of thing happens all the time and is incredibly hard to change once the behavior has been established.