No, it doesn't work 'that' way!
It's ALL personal taste, NO Rules in choosing your Bridal Party.
What if he had 5 Brothers and 2 Sisters....all of age.....would that mean he has to have a "TRIBAL" Wedding Group? Not EVERYONE wants a large Bridal Party. Some, like my Brother, only had a Best Man, the Bride's choice of Maid of Honor, and 2 Ushers, he chose & their Partners, 2 Bridesmaids his g/f chose.
Your son's Best Man and Groomsmen, some call then 'Ushers', are the Guy's he feels he is closest friends with, someone he hangs around with, a College roommates, childhood Buddies, etc... It doesn't necessarily mean his 'Brother'....unless they really are, then fine. Even 'if' that were the case, don't you think the other Brothers will feel left out?
As for his Sister....well, the Bride has the say on HER choice of Maid of Honor and Bridesmaids.
"IF" your son wants his Bro's in it fine, if not it's HIS choice and same for his Bride!