Here is a current Travel Warning issued by the US Department of State for Americans traveling to Nigeria.
http://www.travel.state.gov/travel/cis_ ... w_928.html
Tell your sister that this warning is unbiased and only based on fact. It is intended solely to protect any American who may be traveling overseas.
Unfortunately, in my industry as a fraud investigator, I am convinced that 99% of any activity, event, or person in Nigeria for any reason is a scam that could possibly cause loss of money, property, or even loss of life.