I need opinions!?

Okay so OKbasically, I bought a truck for my mom to drive until I got my license, well it s time to get my license in a few months, and my is still driving my car. My car has 135,000 miles on it and I told her I would buy her a car for 2700 dollars but she dosent want to feel rushed and she wants to keep driving mine for the time being. I m just worried it s gonna get all the miles pu on it before I even get behind the wheel! She keeps telling me no she don t want me to buy her a car.. so I took the keys and locked my car door and said when she decides she wants me to get a car to let me know, I know it s selfish but my sister has a car that is an 2012 car with 60,000 miles and she won t let mom drive and my old 1999 car I is getting drove everyday by her, I don t know what to do!