Nina Sinclair is a Moroi and the half-sister of Dhampir Olive Sinclair. She spent several years in search of her sister after she was turned into a Strigoi in the act of protecting their father from them. She managed to turn her sister back into a Dhampir with her friend James. She is a Spirit user, and seems to be attracted to Adrian Ivashkov.
Like most dhampirs, Olive (Nina's half sister) was the illegitimate child of a Moroi man and dhampir woman. Most Moroi men couldn't be bothered to even meet these kids; Olive's father was a huge exception. He took Olive in to live with him and raised her with her legitimate sister, Nina. He also divorced his wife, Nina's mother, and married Olive's mother. Olive and Nina were very close in childhood.
After receiving training to become a Guardian, Olive was distraught that the Guardians did not honor her request to be assigned to her Moroi family (presumably because they are not royal). She decided to leave the Guardians and protect her family anyways.
Eventually, the Sinclair family was targeted by Strigoi. Olive sacrificed herself to save her family and fell victim to them. Instead of killing her, they chose to "awaken" her.
After news of Lissa's miraculous healing of Dimitri came out, Nina decided she would use her spirit abilities to save her sister from being one of the undead. Nina tracked down her sister in Dallas, Texas. Here, she ambushed her sister to heal her back into a dhampir. Her friend, a dhampir named James, helped restrain Strigoi-Olive so Nina could spirit-stake her. She managed to return her sister to her original state.
After the transformation, the sisters were visited by Guardians Rose Hathaway and Dimitri Belikov. They were later joined by the spirit user Adrian Ivashkov. After some persuasion, Nina's sister, Olive, volunteered some of her blood in the hope it would help protect the Moroi from being turned into Strigoi.
Nina along with her sister where sent to Royal Court, where later on Adrian and Sydney visited. When at Court Nina seems to have developed an attraction to Adrian, despite him being in love with Sydney Sage. Nina repeatedly asks Adrian to go with her to places around Court, and would constantly be talking to him.
When Adrian comes back to Court with his mother, Nina finds him and they end up being drinking buddies. Adrian buys Nina expensive clothes, but Nina treats it as a loan, even though she could probably never pay him back for everything he brought her.
Nina's intentions aren't the same as Adrian, as she wants to become involved with him. One day, when Adrian is asleep and Nina comes in to check on him, he pulls her close and kisses her, thinking she's Sydney in his dreaming state. It is after he wakes up when he tells her that they can't be together. Due to being a spirit user, Nina takes this information a little harder than most people would.
Adrian never got to apologize to Nina about everything he had said as he had left to go and save Sydney from re-education. When Adrian and Sydney arrived back a Court, still in their wedding clothes, it was then she realized he had been in love with a human the whole time. Nina was never really the same around him after that, and their friendship had become strained.
Sonya asks Adrian to speak with Nina to find out why she's in such a bad shape, and Adrian reluctantly complies.
Adrian discovers that Nina had been using an enormous amount of spirit trying to connect with Olive and hasn't been sleeping. Adrian agrees to join her in a spirit dream to see what she means when Nina says Olive is controlling it. When they connect and Olive changes the dream and sends a lava-monster after them, Nina uses a heap of spirit to destroy it, and is yelling at Adrian to use some too to help.
When Adrian leaves to find Sydney, Nina covers for him, using compulsion on anyone who comes by their house into thinking they have seen Adrian, as not to make anyone think that he had left. In return, Adrian agreed to another dream, but Sydney forced him not too. Nina pulled him into a dream anyway.
In the dream, they find Olive, but she changes the scene to when she turned Strigoi. Adrian tries to kill them slowly and effectively, but Nina doesn't want to take her time. She uses a ridiculous amount of spirit to shoot fire at all the Strigoi and kill them. The amount of spirit is too much for her, both because of the high amount and because of the physical and mental toll all this searching has put on her. She collapsed into screams, and ends up in a hospital.
Not long after, Sonya calls Adrian and tell him that Nina is practically comatose and when she wakes up babbles stuff that doesn't make any sense. She says it's mostly due to exhaustion as well as spirit, and should hopefully recover over time.
A few days later, Sonya pulls Adrian into a spirit dream and then brings in Nina. She seems to be trapped in a cell. Sonya tells him this is what she sees in her mind, that Nina is trapped. She doesn't seem to recover by the end of the series, and could possibly be in the same place as Avery Lazar.