Seriously: I am confused why such a small child (she isn't past 3 or 4 years old) was alone at home like that... things are different in Japan? You'd go to jail for that in Canada...
This is manga world, where parents are either never home or so childish their kids needs to be the mature ones and in charge of everything.
Plus, y'know, the whole fiction =/= reality thing :p
Also, I'd say onee-chan's sanity is already gone. XD