How to deal with a sister who doesn't wash her hands or flush the toilet?

My sister literally refuses to bathe, flush the toilet and wash her hands. You might ask: "Max, how do you know she doesn't?" The way I know is that it doesn't seem like she does, so I tell her to, and instead of denying that she did it, she just flat out refuses. And she's not a toddler, she's 13 years old. She's glued to her iPad. I've brought this up with my mom but she refuses to consider an argument between us. All 3 bathrooms in our house are like tainted by her. What can I do to salvage the two years I have left in this house?
How to deal with a sister who doesn't wash her hands or flush the toilet?
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