Phineas and Ferb's relationship

Phineas and Ferb are step-brothers, but they treat each other more like friends. They first met when Linda Flynn married Lawrence Fletcher when they were both around four years old. (Phineas and Ferb Get Busted)

Phineas's Opinion of Ferb
Phineas is Ferb's brother, and thinks of him as a man of action. He once said that "he couldn't ask for a better brother then Ferb." (Rollercoaster) Phineas only mentioned Ferb not talking much twice. (Rollercoaster) (A Hard Day's Knight)

While doing Big Ideas
Phineas thinks of Ferb as handy for thier big ideas, because most of the time, Ferb builds the invention while Phineas plans for it and operates it. Many times, Phineas relies on Ferb to help build the invention. In The Beak, once Isabella found out that Phineas and Ferb were the Beak, she gave Phineas most of the credit, even though Ferb controlled all the weapons and gadgets.

Ferb's Opinion of Phineas
Ferb is Phineas's brother. He treats Phineas more like a freind. Since Ferb doesn't talk much, not as much is known about his opinion of Phineas.

While doing Big Ideas
Ferb often does the building for the Big Idea, while Phineas makes the blueprints. In The Beak, Ferb didn't like Phineas's elements the Beak suit that had to do with "bulgarian folk dancing". That was the only time Ferb didn't like Phineas's idea.