I know it's impersonal, but I've never heard of a kid who didn't love to have money to spend & more personalized gifts are often very difficult but esp. if you really don't know the recipients. Nailing someone else's taste is so tricky. (I was so happy when I finally managed to get my mom to quit buying me clothes as gifts. She knew me well but still missed the mark most of the time.)
I recently sent a check to my cousin's son for his high school graduation & was pleasantly surprised to receive a prompt thank-you note.
Unfortunately, I'm so out of it I have no idea what the cool stores are these days for older kids & young adults, but I'm sure someone can help out there. Maybe an email to your siblings saying you really want to do a little something & what stores do the kids like?