I think that Candace and Phineas's real father is Doofenshmirtz and I have alot of proof to prove it.
1. Linda and Doofenshmirtz dated
2. Both Doofenshmirtz and Candace are allergic to dairy
3. Candace was the only one to beat Doofenshmirtz cup stacking record
4. Candace's middle name is Gertrude which is also Doofenshmirtz Grandmother's name
5. Phineas and Doofenshmirtz like to build things
6. Both Phineas and Doofenshmirtz have the same triangle head shape
That's it for now