How is ferb related to phineas in phineas and ferb?

In the show you can only tell that he is Phineas stepbrother from England and son of Phineas father but then again Phineas mom and Ferb's dad met when they were teenagers.. does that mean that Phineas mom had already been married, had 2 children and divorced by the time they were teens.. and Ferb's dad too? It is extremely confusing I am dying to know.. Thankssss if you can answer :)