My sister is gross and my parents are jerks

Ok well my little sister is really gross she is 10 years old and she doesn’t take showers often, brush her teeth, flush the toilet or even use toilet paper. My parents try to get her to have better hygene but lets face it, they are hopeless. My sister’s hair is greasy and full of knots. We recently found out that she has lice. My parents keep defending her saying she got it from some kid at school but I keep telling them that she started it and is likely giving it to other kids at school. My parents think I’m being mean but because of my sister’s lice I had to cancel a party I’ve been planning for 3 weeks. And now my friends can’t even come over to hang out because they could get lice. Apparently my sister gave me lice because now I have it too… yeah I know that’s really gross and I take very good care of my hair. I mean I used to get lots of compliments on my hair and I was so upset when I found out that I spent 3 hours crying in my room. Some people might think thats overreacting but I used to take pride in my hair and I never had lice before so this is just terrible. Now I can’t sleepover at my friends house or even hug them without giving them lice because it spreads so easily. My sister is ruining my life and she doesn’t even feel bad she is actually glad to see me miserable and in tears. I was so angry I kept saying it was all her fault and my sister said “I’m going to keep not showering until you shut up!” Can you believe it, I’m 15 and she is 10 and she is aloud to talk to me like that. It’s not fair my sister is a brat and if I do anything about it I get in trouble. On top of that she seems proud of her poor hygene. I told my parents that she started this whole lice thing and my dad blamed me. He said that if I cleaned the house more often my sister would never have gotten lice. Neither of my parents think this is my sister’s fault. My mom laughed at me when she saw me crying and said, “Wow you aren’t done crying yet?”. She says I’m too dramatic and she is right but I can’t help it. Everytime I look in the mirror, see someone with really pretty hair or even touch my own hair I feel disgusting and I want to break down and cry. Am I wrong? Is it really my fault? Was I being too harsh? I guess I just needed to vent… anyway thanks for reading.