John Gill Commentary Numbers 2:12

"And those that encamp next unto him shall be the tribe of Simeon: and the prince of the children of Simeon shall be Shelumiel the son of Zurishaddai." — Numbers 2:12 (ASV)

And those which pitch by him [shall be] the tribe of Simeon ,
&c.] Under his standard, and on one side of it; Reuben and Simeon being brothers by the mother as well as the father's side, might well be thought to agree together; and Reuben being the eldest brother, and the eldest of all Jacob's sons, Simeon would not grudge to pitch under him:

and the captain of the children of Simeon [shall be] Shelumiel the son
of Zurishaddai ;
of whom see (Numbers 1:6) (7:36) .