So, wait, Hawke's Warden siblings AREN'T his Warden contact if Bethany or Carver are Wardens?
What the actual ****?
For one thing, having his/her little brother or sister as the amicable contact willing to divulge Warden secrets and with whom he has a steady friendship is infinitely more believable than Stroud, who a Hawke without a sibling in the Wardens met a grand total of ONCE in-game. I guess they could have ran into each other in the ensuing post-game, but they seem a bit too angry-chummy with each other for how little they ever actually met on-screen in-game. Stroud as the contact and Warden pal makes a bit more sense if Carver or Bethany is a Warden, because there's a much deeper personal connection there between Hawke and Stroud; they met in trying circumstances and know someone who is, by all accounts, an outstanding Warden, but still. You don't go to just a guy you know if you also have a close sibling you can go to who knows literally more than the guy does, because they were with you when you fought the ancient darkspawn magister in a secret prison their father helped secure.
For another thing, it would add such a great degree of tension and drama to everything that happens in the Fade. I could totally see Carver giving Hawke **** for helping start the mage rebellion, and I could easily see a Warden Bethany doing the same thing. But it would also make the moment of choosing who has to stay and fight the Lovecraft-Spider that much more affecting, because we're not choosing between an adored PC and that one guy who showed up once or twice, literally within those specific numeric boundaries, and has a rockin' mustache and accent. We're choosing between siblings, both of whom we've played an entire game with (if you got or watched the DLCs that let them get in after Act 1), and who have great potential as heroes and lives to live for, such as they are.
Also, Hawke telling Carver to help the Wardens come back from accidentally almost becoming Corypheus's pawns is a great way to further their own relationship; A Warden Carver will come back to Kirkwall to help Hawke, but here Hawke is putting a heretofore unparalleled level of trust in his/her younger brother, saying without question he can do something that Hawke cannot, that he has a responsibility Hawke cannot possibly assume or overshadow, the one thing Carver always kind of wanted to be acknowledged for. It's just that that moment comes when their lives are irrevocably on the line.
Admittedly, Carver gets more story-awesomeness out being the Warden contact than Bethany does (there was never a huge amount of animosity between Hawke and Bethany, even if Bethany becomes a Warden. Or at least, it was never quite to the extent it was between Hawke and Carver, and it seemed like Bethany got over hers fairly quickly; Carver's getting over the tension between him and Hawke is a story that lasts the whole game by comparison, whereas Bethany gets angry and gets better within the space of the last act).