My younger brother (by 7yrs.) is bigger than me. But I didn't know that until we were adults. I never saw him naked as a teen b/c by the time he was in puberty, I was away at college. And since we're about the same height and weight, I just assumed that we were about the same dick size, too. (Not that I thought about that a lot either.)
The first time I actually saw him naked as a man, he was about 19-20. We were both visiting at home and shared a bedroom/bath. He was toweling off as I was shaving. We were definitely about the same size flaccid.
Years later when he was about 24, he lived with me and my wife for a summer, and he invited his gf over for a weekend. I saw them having sex in the den, and when she shifted positions, I saw his dick erect. Lil bro' shocked me! He was bigger-- not thicker, but longer. I'm 9 inches hard. My guess is he's about 9.5 or 10.
After that, I kind of wish that we were close enough to do a MFM-- tag-team a woman together. NO, I DO NOT WANT TO HAVE SEX WITH MY LITTLE BROTHER, but I wouldn't mind a friendly competition of love-making skills. Who could make her moan louder, orgasm or squirt?