I'm pretty worried because my younger brother is 15 and I'm 17. He is more developed than I am. He has an adult-sized penis, has gotten way taller than me over the past two years, and his voice is sometimes deeper than mine. Why did I fall behind in puberty? Is it normal for my younger brother to be more developed than I am?
Even though you are in the same family and have the same parents, you don't get the same combination of genes from your parents. Your younger brother started maturing a bit earlier than the average, but you started maturing a bit later than average. This isn't uncommon but it is more noticeable when two boys are close to each other in age. Each person grows according to their own schedule. It is annoying when you are the older brother, but things will get back on track in a few years.