I don’t think Cal had much to do with creating Bro. Lil Hal’s existential frustration and loneliness already leads him to doing some pretty horrible shit–and Dirk stresses out about it so much because he really does see that potential within himself. The trend with Cal as I see it is to influence and talk to Caliborn, Gamzee, and Bro, but it only really seems to show the former two reflections of their latter day selves as part of Lord English. It doesn’t coerce anyone.
I think it’s core to the narrative of Homestuck that Caliborn and Gamzee choose what they do, full stop. I think the same is true about Bro, and I don’t think a Bro who grew up in the world–and possibly even with Grandpa and Mom–would’ve taken much convincing, because he wouldn’t be able to have the same relationships with them that he did with his friends. This is especially true if what I suspect about Grandpa is also true for Bro, and he remembers his Alpha life, which I think is plausible given that he’s a Heart player.
But there is one other thing I think sets Dirk apart that matters at least as much as Dirk having had his friends to grow up with, as I alluded to towards the end of this last essay:
Bro didn’t grow up looking up to Dave. Dirk largely escapes his potential as Bro by destroying that potential in himself and living up to a different legacy–Dave’s legacy of Knighthood and service. That he succeeds so strongly that Grandpa remembers him across universes as a Knight implies this better than anything.
Dirk and Bro are inherently different people on such a wide degree that their mythological roles are fundamentally different. Bro is a Prince of Heart, albeit one who’s gone off the deep end and become deeply destructive. Dirk is a Prince of Heart who strives to act as a Knight, particularly in service to a Page of Hope who believes in him and gives him an outlet for that service in tandem. Hence their differences.