Hey now, let's not stray off topic here and challange other people!
boyboy - Your penis may grow as big as your bro's. We don't know, because everybody grows at a diff rate. Just to let you know, 5/6 is the narrow average! So you're already at adult size! But since you're only 15, it may grow a little bit more. I always like it when my Jr. gets bigger
And surenix - I believe it's related to puberty, it's a size question!