I (21M) and my sister (18F) have been living together for just under a year. We initially moved in together as she wanted to take a gap year and escape our small town for a while, and I still had one more year of school left to do and needed a roommate.

Recently, she made a trip back to our hometown, and when she was about to leave was checking her coolant levels in her car, my mom came out and gave her a hand, she also checked her oil. She was floored when she saw that there was no oil in the engine, like none. They immediately brought the car into the town mechanic, my mom chewed her out, then made my sister take her car back home instead.

According to my sister, when she got her oil changed (7 months ago), the mechanics told her that her car had a "slight oil leak" but that it "wouldnt need to be addressed for a while". She says the mechanics told her 1 year, which I highly fucking doubt. Her argument is: "if the mechanics thought it was a real problem, they would have told me so they can sap up more of my money".

I've been known for lecturing sometimes, so I did my best not to by texting:

"I hope you learned something from this", to which she replied with:

"what would I have to learn? that my car is shit? this isn't my fault"

and that really irked me. I don't have the exact convo because snapchat didnt save it, but I basically told her that she did not check her oil at all, she knew about it for 7 months, and she did nothing to prevent it. If our mom didnt bail her out (again) she would have no car, and hell if she didnt find it out then, she could have permanently damaged her engine (we still don't know the damage).

She simply cannot register that this is her fault, and now that she is back in our apartment, I tried to tell her that "I need you to understand that this is your fault, you had a responsibility", but every time I bring it up, she just gets annoyed and goes to her room. My sister has a tendency to dodge accountability, and she still has not learned her lesson. This isn't the first time something like this has happened, my mom has had to pay for my sister's car troubles multiple times on end (something she shouldnt have done imo). I'm hoping when she sees the true damages that it will finally open her eyes.

AITAH? As an older brother I feel like she seriously needs to understand that this is her fault, if our parents hadn't saved her ass again, she would be completely screwed

Side Note: My sister has a tendency to ignore our parents, my mom thought it best that I try to get through to her instead. I genuinely don't know how to go about this, and I don't want my sister to start seeing me as a parent rather than a brother.