The only advantage I can see to making Prince Jason a sibling is making it a little harder for future writers or editors to kill him off.
Heroic Perez Hercules, Champion, Rama, a new Olympian (Achilles) or Orion - writers have virtually deep-sixed them all, without ever seeing how having them around could benefit the WW comic. I think Geoff Johns was right in giving Diana a 'Mr. Wonder', who might drop by, once in awhile, because I think it will help some male readers, who presently don't read WW, to empathize with and embrace the leading lady. Furthermore, it might deter those same writers from making him a cliche brother-villain, when the WW comic might actually be helped by a heroic brother figure, who is part of the Wonderverse. Making him a twin makes him even harder to ignore or kill off; so, while I hate Diana having a male twin...
If it's the only way to keep a Mr. Wonder in the comic, maybe, that's how it had to happen.