I dont think anyone in your situation would be happy, let alone easily stop thinking about it. Her hygiene level is something to worry about, and much more as her actions are affecting the older family members. You just have to calm down a little no matter how hard it will be, and follow here easy. Their mum your sister spoilt them and nothing would be automatic here. So just try to calmly correct here, and clean the much you could.
Hoping she becomes more responsible as she matures.
That's true too, it may take a while for her to take ahold of things but she will find her way, eventually. Thank you, Charles :)