Biologically, no, they are of no relation and never were. As you read the series, your questions will be answered. Carlisle saved each of his "kids" and "adopted them." Since they never age physically as vampires, he has to keep that act up for the humans. Since they all look like they are in high school, they all go to school and then "graduate" depending where they live. Jasper and Alice are a couple, Emmit and Rosalie are a couple, and then Edward finds Bella. None of them are actually brothers and sisters. They have all come under the umbrella of Carlisle to follow his "vegetarian" style of being a vampire. So in order to live in our world, they have to pose as a "family" of adopted kids to this young and caring couple.
They are not blood brothers and sisters. They pretend to be foster siblings so that Forks will never know the truth: they're a family of vampires. Forks High School has a judgmental opinion of them: Rosalie and Emmett are together romantically, and Alice and Jasper are together romantically. But, like I said, they pose as foster siblings. Rosalie and Emmett are actually married part of the time.
Carlisle and Esme are Jasper, Emmet, Rosalie, Alice, and Edward gaurdians since they became vampires.
Biologically they are not related in that way but they live with the Cullens and are known to be siblings except for the fact that Rosalie and Emmett are a couple while Alice and Jasper are a couple.
Mentally wise they all see each other as siblings and love one another like brothers and sisters ( aside for the pairings with Jasper and Alice, and Emmett and Rosalie). Biologically wise they are not brothers and sisters although you could probably argue that since Carlisle changed Edward, Rosalie, and Emmett then it would make them linked somehow since the same venom changed them but then again you can't ignore the pairing, Rosalie and Emmett, as well as Carlisle wife Esme, so it will be a battle not likely to be won haha.
who the F is Emenet, its IMMINENT that you are so wrong.
no but yes tecnally carlisle adopted them because he turned them into vampires
No there not but they have been together so long they just say they are siblings.