>>1Your situation aint as rare as you might think.
Its one of the many sibling problems out there. Its because your the older one that your mother looks upon you as a more experienced adult, in comparison to your sister. Mainly because your maybe believes that she is going to lose her "precious daughter", that she implying all that kindness into your younger sister and jsut spoiling her.
Also, since it seems that your education is going to be messed with, thats just horrible, do what you need to do to convince your mother that what your younger sister is doing is wrong.
If needed, I recommend taking matters in your own hands, or finding a loop hole in your sisters plans and expoliting them