my sister is always lying and i had enough

my sister is 21 and i am 18 but my sister is the most irresponsible ever. today i found out that she lied about passing her second year of medschool (we're half way in) my parents and everyone believes shes in the third year even tho she should be in the 4th year bcz she also failed her first year she tried to lie her way thorught it but she couldnt. (she actually faked everything and we believed her for like a week) shes always been lying about her grades in highschool too (like reaally important exams);

i know my sister has a serious problem she's going to see a therapist tomorrow for the first time she never goes out of her room bedrotting 24/7 escaping reality i know that her lies are not out of malice but im still mad at her

now she also dragged me into this, the only reason i got her to admit this is bcz i promised her im not telling mom but i truly cant lie to her i feel so bad my sister is really a big child i hate her

please give me advice