I'm INTP. My twin brother may be ENTJ. We're actually both older depending on what you call "born" (long story), so I put down eldest on the poll.
My brother and I fight constantly, both physically and mentally/intellectually, even though we're in our late teens. My brother is tall and fast and I'm short (although strong), so he tends to win physical fights. I'm a faster and sharper thinker than my brother, but knows how to get in my head and I therefore often lose arguments, even though I'm usually right. As we get older our fights are becoming more and more mental/intellectual.
In school, we both do pretty good: A's and B's. A rare C. However, I do not have to try hard at all to get good grades. My brother often studies long hours. I can write an A essay in 1.5 hours. He can write an A essay with 5 hours of work. He's actually quite intelligent, particularly in English/History. He just takes a very long time to show it.
Our relationships in 10 years: My skill sets and interests lead me towards cryptologist or detective. Unfortunately,cryptology is a hard field to get into, and even harder to get out of. However, I love codes, so it's definitely on the list. Police detection sounds incredibly by-the-book, so I'd probably be bored by the routine and annoyed by the bureaucracy. Private detection sounds like I'd have more freedom, but the cases would probably be boring. I would need to exercise my intuition and think out of the box for it to interest me, but I enjoy looking into cases on my own time and I am not only learning a lot but it is something I know I enjoy. Sadly, these jobs do not make much money.
For my brother: Stock broker or inventor. My brother has a lot of ideas that he plans out immaculately, and he wants to be rich and powerful. However he is a private person. He wants his own private island, army and fleet of helicopters and cars.
My prediction for the future, if this is how it turns out: My brother is a rich and clever inventor with too much influence. I help him out when he has an issue that involves intuition and quick thinking, and he gives me some money.
I'm kind of looking forward to it.
Does this conform to birth order theory? Not at all. I wonder where we went wrong...