I hate the sibling twist more than anything else in SW. It feels contrived, universe-shrinking, and ultimately pointless.
As to how Vader could provoke Luke (which is actually the alleged reason for including the twist in the first place), he does say the Dark Side is the only way to save his friends. Okay, I’ll admit that’s pretty weak, but I’d let it slide to not make them siblings any day. That said, I’d certainly prefer a better cause of provocation.