Ferb IS Phineas and Candace's step brother.

That means Candace called Ferb her stepbrother, not the usual Ferb she says.

In Rollercoaster, after Isabella is introduced and says 'Hey Phineas, watch'a doin?' she says 'Hey Ferb' and he waves and she says to Phineas 'Your stepbrother doesn't talk much does he?'

there is an episode where candace has a dream whch is perrys dream where phineas and ferb are sent to a gruesome summercamp, in that there is a song sung by stacie called "little brothers" where the whole sequence of ferb being a stepbrother is shown!!!!!!!!!

I have no idea what i'm doing right now xD