You're lucky you're that tall
! Your genetics have been kind to you man, be grateful... Know this, every guy wants a big dick. Most of the time, it matters more to the guy about his penis than to the girl
As for you cousin, he's probably an early bloomer... I started getting upper lip hair at 13, and I thought that was pretty young! Nearly 2 years later, other than that lip-hair, I still don't have a whole lot of facial hair! Puberty doesn't really take a set path I don't think. It's random, I think... I had lip hair waaay before this one kid in Middle School... Now, in freshmen year(grade9) he has all this hair on the side of his face and I've still got the lip hair haha...
Anyway, I doubt you'll be 8" man.. I'm 5'7.5" and 6"(15 in 12days). I think 7.5" is wishful thinking for guys around our size and age
Puberty is a nightmare, filled with "I wish"s.
I think your penis is a tiny bit above average for your age.