The sister 'laughing like a demon' and acting like everything is a joke even though this is clearly the end of the world makes her seem like a psychopath. I don't know if this is what you're going for this early, but my running assumption is that she is
If there are no people around at all, how do they have running water, gas, electricity and access to certain fresh foods? It takes basically no time at all for these things to either shut down or run out when there is no one left to maintain the infrastructure or deliver new fresh goods.