I think the book had dragon in the title not too sure. The older sister I think had ability to control fire I think. They somehow ended up in a castle. I can't remember much of it.
I want to say The Secret of Dragonhome by John Peel, published in 1998.
Melayne and her young brother Sarrow are Talents, hunted by the King's relentless Seekers. If their Talent is discovered, they will be sent to their death. So they must hide...or die.
Melayne and Sarrow find refuge at Dragonhome, the mysterious estate (read: castle) of the shadowy Lord Sander. Lord Sander also has something to hide--a dark and fantastical secret that haunts his every move. In order to save herself and her brother, Melayne must confront forces much stronger than her talent--tempests of magic, desire, and betrayal. If Melayne is not careful, her truth will be revealed.
Melayne must unlock the secrets of Dragonhome. As danger approaches, it will be her only hope.
Melayne has the ability to talk to animals and Sarrow isn't quite sure what his is yet for most of the book. I haven't read the book in a really long time, but I do believe it's revealed that he has the Talent to influence/control and he may or may not have been controlling his sister.
Edit: I just pulled my copy off the shelf and indeed near the end of the book Sarrow's Talent is revealed:
"Your brother has a Talent all right...One of the strongest and rarest I've ever seen. He has the gift of Persuasion...He plays innocent, but he's aware of his Talent...But now that you're aware of what he can do, it won't work on you. He can only Suggest, not Control, thank the good God."
Sarrow betrays his sister and joins the people hunting them. In his words:
"I can control humans simply enough, but Talents are harder. You slipped out of my control without even knowing it. I can't chance more Talents being born."