There's no better research and training program offered to teenagers than their sibling's friends. Here, from the comfort your own home you can study and reflect on how the other gender lives: what they talk about, what makes them laugh, what they eat after school and what they look like sleeping in the basement. I credit my Funny Sister for exposing me to hundreds of hours of "female study" via her friend group. In fact, I believe my first dentist and urologist was little Stacy O, an 11 year-old girl from around the way.
And Stacey O. speaks to your issue: when the study of the other gender turns into hands on field-work-- in your case, hooking up with your brother's buddies. Now in some states I believe you being 17 and hooking up with 21 year olds is illegal. I'm not certain, but I will have my research team investigate this. In the mean time, I'll speak to other issues.
Hooking up with his posse likely has a negative impact on your bro and creates an awkwardness between him and his friends. Friends come over expecting cable TV and your Mom's iced tea and instead they get much more: the Family Jewels if you will.
Maybe your brother plays it off like he doesn't care, but I'd imagine he feels a bit embarrassed, and invaded. Is this inevitable with siblings and their friends? Sometimes, but none-the-less, it's cringy to know one of your friends is hitting the sheets with your sister. Imagine the awkward conversations he will have to sit through.
Joe: Dude, you know who gives the best blowjobs?
(Enter your brother into the room)
Joe: Um..., no one. I don't remember.
Now, of course this isn't always the case. Maybe you're hooking up with a really sweet guy who wants to build a long term relationship, but your question speaks to "hook ups" and plural friendS.
My advice: explore and train somewhere else. Otherwise, it'll likely cause a rift between you and your brother; it'll likely isolate your bro from his pals and the biggest negative: it puts you square in Baby Sister Booty Call Land. And that's a place you don't want to call home.
I dated my best friend's brother when I was 16 and he was 20. We clicked so well. :) But, it didn't end well, as the age gap was just too much - he was at Uni and I was looking forward to the exciting life of travel and school and love I thought I would have.
Seven years later, and though he's been married and now has a child, and I've been to Uni, had a soul-destroying relationshop, spent all my money and now thanks to our arts recession have no hope of finding a job, we've wandered back into each others lives, and I couldn't be happier.
I think what I'm saying is if you think it's love, then go for it. And I mean proper love, not just thinking that they're amazing when you know nothing about them and haven't spent any time with them. :) But if it's just lust, I'd find someone your own age bracket, or wait til Uni/college.
PS As for when they initiate it, they're probably mostly just trying it on with someone they think is young and naive, so if you're not into it, then put a firm foot down and show them otherwise.
Personally, I think it's creepy to hook up with a sibling's friends, no matter the age difference. However, the age difference definitely makes it just a little creepier. I know there are a bazillion girls who would disagree with me here, but I feel like a 21 year old should be able to get girls who are, let's say, 19 and above. Any younger than that, and I have to assume that he's got no game and therefore has to sink to a lower age bracket, knowing how naiive and easily impressed girls your age can be.
And yeah, the fact that you said "friendS" is a little disturbing. I wonder how sketched out your brother would be if you slept with not only one of his friends, but multiples?? Yuck. In conclusion, find someone your own age.