There is a lot here by Bruce and others on the process of growing large (or any other genetic trait) fish. I do not think they are brothers/sisters but some may be related. Most good hatcheries go to great lengths to not sell (or even keep for growout) just brothers and sisters in one group. There is too much genetic risk in doing that so they try and mix them up some by changing brood stock.

That one fish likely has different genetics plus has a superior feeding pattern.