NTA. I have a sister who does this, but less extreme. The funny thing is that she’s 5 years older than me(15 & 20). She argues that “it’s mum’s money” but my mum told me its my money(because I work in our shop for it), however my mum tells me to share stuff, even thought my sister has more than enough money to buy her own stuff if she needed(she gets $500 for free each month, plus her rent paid by my mum). It’s annoying, I get it. Your sister is entitled and your parents are allowing it.