User blog comment:Kittyfire/Random Topic Blogging!/@comment-4005481-20111007204240/@comment-3394114-20111008111643

Yeah, step-half-siblings, because Phin and Ferb aren't blood related. And yes, Dan said himself that Doof is not Phin's real dad. Oh, and there's lot of proof in episodes, don't start me, I'd probably make a long list that nobody would bother to read.