Phineas parents divorced when he was a baby. Leaving his mother with her two kids Candace and Phineas alone.
A year later she got married again and had Ferb, who unfortunately was born with a serious brain infection and became mentally disabled. Which is the reason he barely speaks in the program.
But that wasn't the worst part of the family. Despite the fact that Phineas was a very hyper boy, Candace was crazy and always had illusions of Phineas and Ferb building things such as a roller coaster, building, a skate park, and many other things, and since it was all in her imagination, her mother never had he chance to see what she saw, which is the reason why she never saw anything in the program. Since she had no one to talk to about it, she wrote everything she saw in her diary.
One day, Candace mother got worried and took her to a therapist, Dr. Doofenschmidt who gave Candace a drug to make her feel better.
Unfortunately, Candace became addicted to this drug and started taking more than she was supposed to. Until she was found lying in her bed dead with her diary near her.
After her funeral, Candace mother read her diary and asked Walt Disney pictures to create a program in memory of her. Which they accepted.