Try to talk her into having a professional do the job--it will be much cleaner, safer and less painful that way. If she insists on doing it at home, though, disinfect the area with rubbing alcohol before doing the piercing and use a new (i. e. never-used) needle. Wear clean, never-used latex gloves at all times.
DO NOT do this for your sister - or anyone else! Now, please don't think I'm against piercings. I'm not. I have 24 piercings, all healthy. To be safe you need to go to a professional using sterile equipment. DON'T do it at home, get a pro.
Amatuer body piercing at home is never a good idea. The risks associated with piercing (injury, infection etc) are too great. Your sister should go to a trained professional.
For a home naval piercing you should have a 14g needle. Make sure that the needle is sterilized before using it. Also make sure that you wash your hands before starting the piercing process. It is also a good idea to wear gloves during the piercing to minimize the risk of infection.
Lets see... you will need a needle or pin, some kind of antiseptic, and the naval ring she wants to wear. You can use ice to numb the area so it is not so painful. Also, she may not want to take any blood thinners, like asprin, for a few days prior to the piercing.