I remember reading one called "Bigger Brothers" on Coiledfist.org a few years ago that sounds like this. The youngest brother (Brady or Brody) was quickly catching up to the middle brother in muscle and height by the time the most recent chapter had been completed. They were both pushing like 6'7". I kept hoping for another chapter, but the story was never continued. Then it disappeared altogether , and I wished I had saved it somehow because it was very hot.