How to tell my sister to stop putting me down?

i have a twin sister and she has been very annoying. a few weeks ago, i told her that i want to become a pharmacist, and i'm aware of the challenging classes that i will take in the future. she goes on and bring me down. she said " you won't make it to pharm schools since you're getting old and do not have a science background" i took some sci'm 24 years old by the way. she said the family might need my help financially right now. my parents are getting old, but they do make money somewhat (we're immigrants so we don't make a lot of money). she drives me nuts, i don't know how to shut her up. we're taking a summer class right now and we have to share the same car and she just won't shut up. i don't know what to do. she brings me down every time we talk about my career. i told her to stop talking about my career b/c , really, it's none of her business. she said it is her business b/c i'm going to make my family poor by staying in school too long. i told her that i will make a lot of money when i graduate from pharm school. while i'm still taking prerequisite classes, i can work as a pharm technician to help my parents out financially. by the way, she got accepted into a nursing school and will attend this fall. she is so annoying! she said i will start a career later than her b/c it will take me at least 6 more years to finish prerequisite classes and pharm school. oh my god. i can't stand her anymore. i try to avoid talking to her when she talks about pharmacy to me, but we're in the same car!!! I can't escape anywhere. by the way, we have to use the same car to save money. i know some of you might tell me to get a new car. what should i do???