my sister/school

so my moms been thinking of sending my sister back to school(we're homeschooled),and my sister did want to a while ago,and i guess still does,but my main concern is money,obviously a school uniform,lunches and books cost money.a pretty big amount of money,plus its not gonna be for that long,i think,maybe two years,which i guess is kinda long but idk,i don't think it justifies it,the main reason my moms thinking of sending her back is because of her attitude,she never wants to do work,sometimes wakes up hours late,which she should obviously be getting up on time,and also when i point out expenses,which again is my main concern,she just says "so?you're not paying for it",like duh but it still affects me,we rely on our parents for income cuz we're teenagers,ya know like for food,i mean she doesn't get that cuz she's irresponsible with money. i don't really know what to do because i don't really have a choice here cuz its between her and my mom,all i can really do is give my opinion to my mom cuz my sisters not worth talking to cuz she just gets offended.