This is the list of possible familial relationships which appear in Phineas and Ferb.
The Missing Parents of the Flynn-Fletcher FamilyEdit
Major Monogram might be Isabella's father or stepfather, in actual Danville universe. Monogram's wife is all about Mexico and stuff. In "Doof Dynasty", Major Monogram is also depicted as Isabella's father. However, Isabella met Major Monogram many times without mentioning that he's her father.
Major Monogram might be Candace's biological father, along with Phineas Flynn being Dr. Doofenshmirtz's biological son, which explains the special relationship between Major Monogram and Dr. Doofenshmirtz.