Siblings of many species vary in size. The smaller one may have nothing wrong with it and will be a happy, healthy tortoise. In fact we raise lots of Sulcatas and last year's hatchlings mostly grew at the same rate, but one is noticeably smaller than its brothers and sisters... but eats well...
Our two males are approaching 100 pound monsters with the female slightly less. Big. Big.and very active, as all three come running (as fast as big tortoises can move) when they see me with a big tray full of 15 pounds of assorted greens mixed with two kinds of tortoise pellets. No conflicts...
We have very large Sulcatas living together... 2M and 1F. All happy and no bullying. There are 4 "caves" for them to sleep in and often all 3 sleep together. When I place their large food tray in their paddock, no one ever crowds the others for a spot, but the last one to the tray walks...