swine

Yup, they're from a blended family. As far as I know Ferb's mother isn't revealed or mentioned in the show, idk about Phineas and Candace's father. There's a theory that Doofenshmirtz is Phineas's father because they have a similar head shape. Whereas Ferb has his dad's head shape.

I've seen that theory around, though it wouldn't make sense since Candace and Vanessa are about the same age, and Doof is many things but he's definitely not a cheater.

That's a good point actually. If I recall correctly there was one episode where Phineas and Candace's mom had a flashback to a lame date with Doof but that was before Candace was born, and yeah I don't think implied cheating would be something they'd put in a kids' show, and it doesn't fit either character anyway.

Yeah, that ep is when he actually decides to rule the Tri-state area. Also, Doof has met Phineas and Candace on a few occasions and doesn't recognise them, so it's probably not true, but a fun theory nonetheless.