[from_another]

I have read an SF book series five or six years ago, I think, and would really like to read it again, but I can't remember the title or the author of the series. So, there were seven books and the first book opens with this boy and a girl who are siblings living with their father on a dragon farm, or something like that, in an imaginary world, and one day these people come and kill their father. The whole farm ends up burning and the boy and the girl escape riding a dragon, if I remember correctly, and spend the entire series fighting with the people who killed their father. At some point in one of the books they get to this city that looks like it's built in a jungle, with these elevators and stairs that look like they're made from lianas and the enemies they fight throughout entire series attack the city and then the boy and the girl lead the defense...
It's a pretty poor description, but I can't remember much more.
If you have any idea of what I'm talking about, please post a response.
Thank you very much!! :)