Why is my mother insane about me turning 21?

I turn 21 in a couple of months. Around the same time that I go back to school and my younger sister starts college for the first time. Dinner started out just fine until my parents gave some pre college advice to my younger sister and then my mom found that as an opportunity to call me a drunk idiot. She then proceeded to question me as to how many times I had been drunk in the past year (zero by the way) and then continued to yell at me for having a social life. She told me that at school I should only be focusing on school and not going out with friends, in fact she even suggested that I get rid of my friends because they're worthless and will probably stab me in the back (take it my friends are 4.0 students like myself and striving towards different career goals). Before I even had the chance to realize what was going on, she then told me that if I take a drink of alcohol then I'm going to die because alcohol kills people. I then calmly tried to tell her that I was responsible and had never been drunk before and don't plan to (I am telling the truth). She didn't believe me and told me I was an idiot that didn't care about anything that she said and that I was going to end up dead in a ditch before 30. So I told her a little more angrily this time that I wasn't an idiot and I know the consequences. She then turns around and says that I constantly say I know everything and that I should just shut up because I don't and then she proceeded to tell me a story about a homeless person who never drank any alcohol and then went on to run his own business. Because according to her alcohol + me turning 21 = complete failure and how I will never be as good as my younger sister, of course my dad sides with her because he doesn't want to upset her. Now take it I'm a pretty smart guy in both books and common sense and I know the consequences. But what I don't understand is why my mother is so adamant about me dying when I turn 21 and why I should only focus on school and fore go a social life. Why am I never as good as my younger sister in the eyes of my mother?