From the Phineas and Ferb original TV soundtrack
Lyrics:
Little brother
I remember when you first came home
Then came another
Little brother of our own
Even when you break my toys
You will always be my little brothers
'Cause you're younger, we're related, and you're boys
Even when you're making too much noise
You will always be my little brothers
'Cause you're younger, we're related, and you're boys
Little brothers
Little brothers
Little brothers