This is the Faerie Path series by Allan Frewin Jones. There are a total of six books in the series, with the first having been published in 2007, and the last in 2011.
This review of the first book, The Faerie Path (2007), provides a decent overview of the plot.
All little girls with even an ounce of imagination have wondered what it would be like to have wings and be magical. Anita Palmer gets to find out, up close and personal. She is mysteriously transported out of modern day London into the world of Faerie, only to find out that she is herself the long lost seventh daughter of the fabled king Oberon himself. Unfortunately, not everything is gossamer wings (losing them is a rite of passage for younger faeries) and magical tapestries. Getting to know and love her new family doesn’t stop her from missing her old earth-bound parents. And while her escort in Faerie appears both handsome and caring, she just can’t make herself comfortable with the idea of romance and eventual marriage to Lord Drake. Can she really trust him? In a well-paced style that will communicate with today’s readers, Jones relates well the emotional turmoil that all adolescents experience, be they faerie or human. The ending cleanly sets up a potential sequel for further adventures in the near future.
This reader review of the first book mentions some of the other elements you described, such as the protagonist having a dream about flying, her sisters having unique gifts, a metal called Isenmort, which is poisonous to her kind, and black stones that allow her to handle metal.
Five hundred years ago, on the eve of her wedding to Lord Gabriel Drake, Princess Tania of Faerie, with the encouragement of her sister Rathina, explores her ability to travel between the Faerie Kingdom and the Mortal World. Almost immediately, her now-mortal body catches the black death and dies, but her spirit, being Faerie and thus immortal, is born again over and over again, waiting for a chance to journey back to Faerie. (One has to wonder if all her names over the centuries were anagrams of Tania?) Her latest incarnation is as Anita Palmer, who's about to turn 16, and who's fallen in love with hunky Evan Thomas, the new boy who appeared just in time to win the role of Romeo in the school play to her Juliet.
Evan has planned a date wherein he's going to tell Anita the truth of her birth and parentage--but an accident happens to the boat they're in, and they both end up in the hospital. Evan disappears mysteriously, and Anita anonymously receives a strange book that tells the story of Tania, princess of Faerie. She also experiences a strange dream where she flies over Hampton Court Palace and sees it as a dreary waste. Weird. Even stranger is when she sprouts, then loses, wings. Top it all off with her being lured from the hospital balcony by a strange man who takes her hand and leads her to...no, not Neverland...the Faerie Kingdom. The mysterious man is Gabriel Drake, the person she left at the altar five hundred years ago, and who has now figured out a way to come and rescue her. Things have seriously deteriorated in the Kingdom of Faerie since her disappearance: her mother, Queen Titania, supposedly drowned just after Tania disappeared, and the depression causes by the loss of his wife and daughter sent King Oberon into a 500-year-long downward spiral that effects everyone in the kingdom until Gabriel pulls Anita back through the portal and lets everyone know that she has returned. Yay Gabriel!
Turns out that Evan was simply Gabriel's manservant, who had been enchanted and sent through the portal to lead Gabriel to Tania. Anita feels betrayed, and spends a goodly amount of time refusing to listen when Evan tries to explain his part of the story.
Tania doesn't remember anything of having grown up in the Faerie Court, so there's quite a bit of time spent on getting to know her six sisters (she's the seventh daughter, which is what gives her the power to walk between the worlds): Eden, who has disappeared into her studies of the Mystic Arts...but whether it's good or bad magic, no one seems to know, since she hasn't been seen in years. There's Cordelia, whose special gift is animals; Rathina, Tania's closest sister and lover of horses; Zara, whose gift is music, Sancha, the scholar, and Hopie, the healer.
Anita spends her first couple of days in Faerie convinced that this is the world's longest and most convincing dream; it's only when she bursts back and forth between the worlds a couple of times that she realizes the Faerie Kingdom really is a separate reality. (The Faerie Court is held in the same place as the location of Hampton Court Palace, so when she travels between worlds, she seems to end up there the most). She is desperate for at least a short trip back to the Mortal World to bid goodbye to her parents there; but at first, she has no control over her travels from place to place. By the time she gets it figured out, it's almost too late.
Edric (known as Evan in the Mortal World) finally gets her alone long enough to tell her the truth about Gabriel Drake's plans--that he wants to marry her in order to harness her power to travel between dimensions and master the use of Isenmort (iron, which is poisonous to faeries) and use it to overthrow Oberon's court and take over the Realm. Rathina leads Gabriel to where the two of them are, and as a result of Evan's confession, the evil Drake seals upup his former servant in an Amber Prison (a ball of amber that floats its way down into the dungeons under the palace, where he will be alive and bored out of his mind forever). Tania, filled with remorse at how unfair she's been to Evan, determines to go back to the Mortal World and steal a sword from Hampton Court, then bring it back to Faerie and use it to shatter Evan's Amber Prison. She is almost captured there by the night guard, but is rescued by the mysterious Eden, who guides her back through a sort of Rose Window in the cloister that acts as a portal between worlds and also allows her to bring the sword too.
They hurry to the dungeon, and on the way, Eden confesses that their mother didn't really drown: after Tania's disappearance, Titania convinced Eden to send her through the portal to look for her daughter, but the Queen never returned. Eden has hidden herself away, overcome with guilt, for the past 500 years (and her hair turned white too!). It was she who allowed herself to be blackmailed by Drake into opening the portal so Evan could be sent through to find Tania. But Eden has decided that the time has finally come for her to come clean and allow the truth to be made known.
Eden can't enter the dungeons, but Tania does, and eventually finds Evan's prison and destroys it. He is injured and it's all she can do to get him out of the dungeon; when they run into Rathina in the hall, she agrees to take them to Eden's room, but instead they end up in the Hall of Light, the place where marriages are performed--and where Gabriel is waiting for them. Rathina has betrayed her again! Gabriel is determined that the marriage is going to take place right now.
A voice comes into her thoughts, reminding Tania to think of the rhyme that details the 7th daughter's powers: One alone will walk both worlds/Daughter last of daughters seven/With her true love by her side/Honest hand in true love given. She realizes that if she agrees and takes Gabriel with her into the Mortal World, it will destroy him because he ISN'T her true love--Evan is. Bam! His plan foiled, Drake is breathing out great threatenings when Oberon appears and banishes him forever.
Tania realizes that the voice she heard in her head is in fact her mother's voice--and she also realizes that Titania is still alive, but trapped in the Mortal World and unable to get back to the Portal (Tania is the only one who can travel freely between worlds, without the aid of the Portal and the Mystic Arts). Who else could have sent Tania's long-lost Soul Book to her at the hospital? The story ends with Tania being commissioned by Oberon to travel back to the Mortal World to look for her mother...and guess what? The king provides her with two black stones that will allow her to handle metal--so she grabs Evan's hand and pulls him through the portal as well, and they're on their way to new adventures! Stay tuned for Book 2. And books 3, 4, 5...
This reader review of the third book, The Seventh Daughter (2008), mentions the protagonist's biological father being imprisoned within bars of Isenmort.
Rescuing the Queen was easy, now Tania has to save the entire faerie realm. Now they must rescue King Oberon if there is to be any hope of winning the war. He is bound in an amber prison surrounded by bars of Isenmort, which will make his release very difficult, if only they knew where to find him. Tania, Edric and Tania’s two sister Cordelia and Zara set off on a quest to Ynis Maw, the end of the faerie world. They hope to find King Oberon, release him from his prison and join his power with Queen Titania’s to over throw the Sorcerer King. The journey is far and there are many obstacles to face. Will they rescue King Oberon in time and lead them into victory?
