Here’s the thing - why?
You look at studying a “grind”, so why are you looking to subject yourself to four years of grind? The “elite” colleges aren’t paradises, where one walks through gardens with flowers and butterflies, enjoying balmy sunny days. They are places in which the kids who genuinely enjoy what you feel is a “grind”. They are places in which you will have to continue to “grind” day in and day out for four years. You should really find a college in which you will be able to blossom, study at the rate which fits you, find classes which you will enjoy, and activities which will stimulate you, etc.
Your entire reason for doing well in high school seems to be “I need to work hard to get into an ‘elite’ college”. That isn’t the type of attitude of a person who would thrive in a highly competitive atmosphere of most of the colleges you are mentioning. Your attitude needs to be “I need to work as hard as I can, and do my very best, because that is what I enjoy doing”
First, every year it gets more difficult to be accepted to any of the “elite” colleges, as more and more students apply, and the “arm-race” of GPA and ECs intensifies. So it was easier for your brothers that it will be for you. Exactly how your brother ended up in MIT is probably related to other things that he did, rather than his high school, since MIT really accepts students mostly based on accomplishments rather than on legacy, prestige or high school, or donations. You should ask him.
Can you get into an “elite” college? Yes. Should you? Probably not, unless you start feeling more comfortable with working hard at academics.
The good news is that there are literally hundreds of excellent colleges at which a student who has a 3.4-3.7 GPA will truly enjoy themselves. There are amazing Liberal Arts colleges at which you’ll can find “your people”, large universities at which you will find both classes and activities which you will enjoy.
A 3.7 GPA is not “average”, but quite a bit above average - the average GPA of graduating high school students is about 3.0.
PS. If your brother was accepted to MIT, Princeton, Columbia, HMC, and Cornell, I find it difficult to believe that he got mostly B and B+ grades for his first three semesters, even if he had the connections of Exeter. Moreover, although a person with a GPA of 3.7 has a small chance of being accepted to any one of that list of colleges, I do not see that a person with a 3.7 GPA would be accepted to all five of those colleges, even if they graduated from Exeter.
Upon checking, it seems that Exeter does not consider freshman grades for GPA (they’re all pass/fail), so a single semester with a one or two B or B+ grades could leave a students with a GPA of over 3.9, which would make them competitive for all of those colleges, especially if they had other great accomplishments.
Unfortunately, unless your high school does not consider your freshman year for GPA, you are not in the same situation as your older brother.
However, once again, you are competitive for a long list of excellent colleges. By all means apply to a couple of reaches, but acceptance rates to all of these are already very low, and they are even more slim for you.
On the other hand - are you a legacy at any of those colleges?