It is implied that Joseph had arranged where each brother would sit. You can make clear the implied information. Alternate translation: "The brothers sat across from the man, according to how he arranged their places" (See: [[rc://en/ta/man/jit/figs-explicit]])
The "firstborn" and the "youngest" are used together to mean all the brothers were sitting in order according to their age. (See: [[rc://en/ta/man/jit/figs-merism]])