Shadowland is the first book of this series (there are 6 total). It introduces you to the main character, Susannah (Suze) Simon a sixteen year old girl. She explains that she is a mediator-someone who can see, speak to, and touch ghosts and her job is to help these ghosts "resolve their unfinished business with the living"
Her dad died when she was young, and that is when she found out that she was this mediator. She had never really understood it before until her dad had come back to explain it to her. So now, she's sixteen, and she's moved to California to be with her mom and her new step dad and his sons. The house they are living in is old, and when Suze moves into her new room, she finds it is already occupied by a "hot ghost" named Jesse. Throughout the book, Suze deals with sharing this room with Jesse.
When Suze goes to her new school, a Catholic school, with a "Father" as principal, she finds that he too is a mediator. Since the school was very old, Suze had expected to be very busy trying to move ghosts on to their next life, however when she found out that Father Dominic was a mediator too, she realized the reason for the lack of ghosts.
The main part of the story is that a girl from the school has died, and she is seeking revenge. She was one of those mean popular girls I guess you could say. If I remember correctly, I think the dead girl is in denial that she is dead and Suze and Father Dominic are trying to get her to move on, but she has a temper and is causing havoc on the school because she doesn't feel like people are grieving enough and she doesn't deserve to be dead.
I don't really want to get into it more, and possibly ruin the book. However, along the way Suze has more encounters with Jesse and I think she tries to figure out why he has been around so long while dealing with the crazy dead girl in the process.
I think your neice will like this book, it has a combination of action, romance, and it's about a strong female character =) Hopefully you won't be ridiculed by your sister...Good luck!
I'll leave you the link for Meg Cabot's website where you can read the first chapter of the book and peek at the rest of the books in the series.