Helping a child to stop crying can be a very frustrating task.  You don't mention the age of your niece or the circumstances that surround her crying episodes however it might help you to know that there are strategies that would help you to decrease this behavior.  www.parenting.org is a website that allows you to search a particular topic and to read articles that correspond to various age ranges.  In addition to continuing to look for answers online, I would also direct you to a pediatrician for guidance.
If you are interested in further assistance on this particular problem and would be willing to provide more details, please feel free to write to a counselor at hotline@boystown.org.  Counselors are available 24 hours a day, 7 days a week.  You can also call the Boys Town National Hotline at 1-800-448-3000 to speak to a counselor directly.  
Thank you for writing to www.education.com with your question.

Boys Town National Hotline
1-800-448-3000

I'm sorry to hear about her crying.  That must be difficult and frustrating for you to see her suffer.  Have you determined whether she is crying because she's hurt, uncomfortable, scared or hungry?  If she is crying because she physically needs something, the best approach is to determine what she needs and give it to her.  If she is crying simply because she wants something, there are a few ways to constructively discipline a child.  First, explain to her that if she wants something she should ask, and you will decide whether she maybe granted her wish.  Or, you could tell her to write you a note of the top three things she wants and give her at least one from the list.  I hope these suggestions work well for you.  Take care!