Probably the "The Secrets of the Immortal Nicholas Flamel" series by Michael Scott.
The Warlock, 5th book of the series has some textual evidence, but I am assured that it exists throughout the books.
From this site's summary (emphasis mine):
Josh is one of the main characters in book. He is Sophie's twin brother and one of the twins of legend. He has a pure gold aura, and when he uses it, the smell of Oranges fill the air. Josh and his sister split during the story, each taking one side. To Josh, he picked the correct side, but to Sophie he didn't. They become reunited at the end of the book. Sophie Sophie is also a main character in the book. She is Josh's twin sister and is the other twin of legend. She, unlike her brother, has a pure silver aura, and when she uses it, the air fills with the smell of vanilla.
A summary from Goodreads of the book (bolding theirs):
The twins of prophecy have been divided—the end has begun.
Although their ally Dr. John Dee has been declared utlaga, Machiavelli and Billy the Kid will follow the plans the Elders have laid before them: they will loose the monsters of Alcatraz on the city of San Francisco, thereby triggering the end of the human race.
The Shadowrealm that Scatty and Joan of Arc have entered is far more dangerous than they could ever have imagined. And they haven't landed here by chance—the warriors were called for a reason. So were Saint-Germain, Palamedes, and Shakespeare. The group was summoned because they must travel back int time to Danu Talis to destroy it. For the island of Danu Talis, known in humani myth as the lost city of Atlantis, must fall if the modern world is to exist.
The end is finally near. Josh Newman has chosen a side, and he will not stand with his sister, Sophie, or with the Alchemyst, Nicholas Flamel. He will fight alongside Dee and the mysterious Virginia Dare.
Unless Sophie can find her twin before the battle begins, all is lost—forever.
And, from Google Books:
Her face moved left and right as she sniffed the air. "You can still smell the magic."
"Vanilla," Isis said.
"Orange," Osiris added.