How "little" sister are we talking about? If she's young enough; your parents should have made sure she'd take care of it - well, actually, in my opinion should have made sure she had read up on the care involved.
If she's old enough to know and do better, sorry, some people think the idea of a pet is so neat, but when it comes down to the nitty-gritty of the care involved, they're no longer interested. :/