Car Buying Decision Argument

This is not for me or not even about me. It's a situation that my man and I got into because of her sister's situation. We did not come into a fight, but we were almost there. Good thing I apologized and he understood. We're calm now.
Their parents is in their home from abroad. Complete family. They also have a big brother that came home days before their parents from Singapore. As always, they do go roam around to bond, eat here and there and have fun. Yesterday. This was only in yesterday. He texted me before going to bed, "We are planning to buy our sister a car." I'm like. WHAT? Why?
Then I asked plenty of questions. He answered. I know upfront he knows that I'm not supportive with all of it. His sister is 15 years old and never drove before. Never. 
(Warning: I am biased. My opinions here  might be because of my previous situation and can have a point somewhere or not at all.) M is me. He is N.
M: Why are you going to buy a car for her?
N: For college. They need to have a car.
M: Why all of a sudden, there still two more years before she's on college.
N: So that on her 18th, It'll be done, all paid.
M: She does not even know how to drive.
N: We'll teach her.
M: Automatic or Manual?
N: MANUAL.

Let me stop here before I continue the conversation we had. You can brag knowing how to DRIVE A STICK. But having a Automatic Transmission car is the same as having a manual if you know how to drive both. Biggest difference, it's easier on a manual in a third world country because of the traffic. I know how to do both but I don't make people who do not know Manual feel any less of a driver. Because to tell you the truth, as long as you know both IT DOES NOT MATTER. It's how you freaking drive. That's the bottom line, not how beautiful or expensive your car is.
M: Ok. Who's going to pay for it?
N: Dad put a down and I will continue paying for 2 years. My brother will "help" as well for the monthly dues.
M: Did they talk about this (sister and parents) before hand? It all seemed rush.
N: (I forgot if he answered since I asked a lot, as far as I remember, It was really a rash decision.) It's done yet. We're going to look tomorrow after mass to see and test drive.
Today. They went to church and dealers that the drive by after that. He texted me when they got home.
M: Did you buy it already?
N: No, we only passed the paperwork.
M: (In my mind, the hell.) No, but you already passed the papers? That means it's already processing and you bought it!
N: We chose this and that compare to here. All cute. Hatch back.

He started saying thank you for the support. I can feel it so much (sarcastically). I said I'm sorry I can't lie. Next time, I'll just have to shut up and don't ask anything. I'm not like my man who will keep his mouth shut if he knows we're going to have a fight. Especially if I know stuff regarding buying car and freaking driving. I feel bad to not be 100% supportive about what they did. That's me. But then I remember, it does not involve me. I don't need to stress myself out. I apologized and told him "Let's stop this conversation, I'm sorry." And we both moved on. I will try my best to not say anything unsupportive (if that is even a word) anytime soon. It will be hard, but I need to just shut up.
Now, this is about me.
  1. First thing first, her sister does not know how to drive. And you're going to buy her a car? My mom said, it's not having a car, you also have to think about accidents. Don't get me started about that.
  2. She's 15 years old. In America, it's normal to buy you children at 16. (I don't agree with this though, it's spoiling them at a young age). In our culture, unless you're rich, you don;t get a car until you buy it with your work money around in mid 20's. Yes, I do compare myself when I was in her age. I was already driving in our Manual car back then and never came to my mind to have my own one.
  3. She's not even in college yet and will not be using until she got her license. I got my car with my own hardworking money when I was 18 because in Arkansas, there are no public transportation. And I drive an hour to and an hour to home so a must. My man explained that yeah she needs it for college. But damn, there's two years left! 
Yes, I compare it to myself that's why I feel not supportive. I don't know if I really need to apologize, maybe not being supportive enough, but that's how I really feel. If my man or anyone in his family would read this, this is my honest opinion. Jazzy (sister) deserves a lot since she's the best in her age that I know, I just hope she'll be safe. Learn as fast as possible because having a car is no joke, especially if you're a beginner and not let a free car get into her head. Be safe. That's all it is.