My brother is 7 years younger, so we went though puberty at different times. One time, I was visiting home when he was about 16. While he was showering I used his towel to finish shaving. And when he pulled back the shower curtain, I saw him flaccid-- probably 5", a little thinner, but otherwise similar to mine, and probably impressive for a 16 year old. That same week I made sure to towel off in front of him just to make things even and in case he was curious about me too.
Ten years later, my brother stayed with me and my wife for a summer. I caught him and his gf having sex in the den. She was on her knees giving him a bj. When she pulled back, I saw him hard. My little brother is a full grown man with close to 10"-- bigger than mine, but I have more girth, esp at the base.